Understanding Addiction
Methamphetamine addiction doesn’t discriminate, it doesn’t know any boundaries, it has no fear or favour.  It is a drug that consumes the user and extinguishes their personality and character, replacing it with a confused, delusional being that is a slave to the addiction that created it.
Why is it that this drug, like no other, can take over 9 out of 10 people in a matter of weeks?  What is it about methamphetamine that causes such devastating and destructive addiction? 
Addiction is a disease of the pleasure-producing chemistry of the brain.  ‘Drugs’ of abuse all share in common the ability to activate the chemical pathways in the brain, that when active, yield feelings of well-being, pleasure, and euphoria.  By over stimulating the pleasure system, neuroadaptive changes occur, which damage the normal experience of pleasure.
Methamphetamine is a highly addictive synthetic stimulant, that triggers the release of large quantities of the chemicals in our brain responsible for pleasure, causing a euphoric high which may last for between 4 and 24 hours.

Click here to view "nerve cell function" diagram.

At the heart of addiction to methamphetamine is the level of destruction to the part of the brain central to, and essential in our survival, since time began.  In short all organisms possess pleasure chemistry in the same mid brain region, activated by the same stimuli.  It is pleasure that reinforcers the behaviours which are cornerstones of survival.  These ‘natural reinforcers’ include the pleasure derived from food, sex, and social contact.
The term ‘reinforcer’ derives from the desire to repeat stimulus.  There are two types of reinforcers, positive and  negative:  A positive reinforcer leads to repeat behaviour because the stimuli feel good; a negative reinforcer leads to repeat behavior because it diminishes a negative stimulation.  Both positive and negative reinforcement play large roles in the development of addictive disease, and influence the behavior of people with addiction. 
In addition to survival-based behaviors like food, sex and social contact, the pleasure pathways of the brain are activated during states of ‘enjoyment,’ ‘interest,’ ‘motivation,’ and ‘reward.’  Individuals with disease of the pleasure chemistry suffer not only from loss of pleasure from food, sex, and social contact, but experience  difficulties with the other reward-influenced behaviors, i.e. they lose enjoyment, interest, motivation, and find their experience of life non-rewarding.  With methamphetamine this can be extreme.
Key to understanding methamphetamine addiction is understanding the biological transformation that occurs almost immediately, as neuroadaptaion develops, leading to the inability to feel and experience normal life pleasures, many of which are central to survival.

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Neuroadaptation
The neurotransmitter dopamine is the main brain chemical associated with the sensation of pleasure.  Normal life pleasure has been measured as generating around 100 units of dopamine, with sex being around 200 units, and cocaine, a very similar addictive amphetamine to methamphetamine being around 400 units of dopamine.  Methamphetamine far exceeds this with between 1200 and 1250 units of dopamine being produced, flooding the reward pathways with supra-normal concentrations of this neurotransmitter. 
As with all brain function, this over stimulation causes a compensatory response.  This is known as ‘neuroadaptation’, the process by which the brain adapts to high concentrations of neurotransmitters released by drugs.
Neuroadaptation is the brains natural defence against over stimulation by causing a number of physiological changes, ultimately leading to destruction to the receptors and receptor function, and with prolonged use, the nucleus of the nerve cells themselves.
The receptors are the parts of the neurons (nerve cells) in the reward pathways of the brain that receive the neurotransmitters and trigger the resulting pleasure.  As the receptors begin to die away and receptors function becomes disturbed due to over stimulation, the brain’s ability to feel pleasure begins to reduce immediately.  So it is that the process by which the brain adapts to high concentrations of dopamine and pleasure (neuroadaptation) ultimately diminishes the user’s  ability to experience normal life pleasure without stimulation  through the drug.    
In other words, as the brains ability to feel pleasure at levels up to 12 times higher than normal is destroyed, without stimulation through methamphetamine use, the user experiences life as boring, flat, uninteresting, unrewarding and unpleasureable.  Instead of pleasure and euphoria (maximum pleasure stimulation) the methamphetamine abuser experiences anhedonia (Loss of ability to experience pleasure) and dysphoria (a horrifically negative sensation – the opposite of euphoria).
The reason methamphetamine is such a deceiving and addictive drug in simple terms, derives from the fact that the high levels of euphoria are accompanied by feelings of invincibility, of seeing and perceiving things more clearly than ever before.  The user is flooded with the high levels of pleasure chemicals which at their most basic level are there to reward and reinforce the very behaviour that has caused them, so the user is tricked into feeling overwhelmingly in control and their drug use is positively reinforced, all while their ability to function without it are being destroyed .

Click here to view "reduction in dopamine function " diagram.

Tolerance
As neuroadaptation develops, it leads to a phenomenon known as ‘chasing the ghost’, a state borne out of tolerance.  Tolerance occurs as the brain becomes more and more conditioned to higher levels of pleasure, resulting in destruction to receptors and receptor function, and as a result there is a need for higher levels of the drug or more pure forms to generate the same level of pleasure as was experienced with much smaller doses of methamphetamine earlier.
In short, the first high creates the ‘driver’ which is locked into the biological centre of the reward pathways, something the user will seek to replicate or ‘chase’, but are unlikely to achieve as neuroadaptation and tolerance occur.  As the higher levels and frequency of use grow, the cycle becomes more and more inescapable.  Moreover, where a meth user develops tolerance they will experience a degree of withdrawal when they don’t have the drug.  In general, the user will feel as low as they have felt high.   
In the early stages of addiction, users often feel invincible, and denial is common.  It is not until they try to remove the drug from their lives or they are unable to get it, that the realisation of their addiction becomes apparent.  That is, that the brain has adapted to high levels of stimulation, while reinforcing a feeling of invincibility and reward with the user, and it is not until they are without the drug they realise how reliant they are on it, to the degree they experience intense biological cravings and overwhelming feelings of dysphoria without it – they are addicted! 

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Addiction treatment
Four interrelated symptoms define addiction:

When confronted with cessation of methamphetamine use, users rebound towards dysphoria (Characterised by the inability to mobilise energy, loss of ability to feel pleasure, and the appearance of intense drug hunger and craving).  The users’ normal mental state changes from feeling well, to feeling flat, empty, unpleasant and dysfunctional, heightening significantly the risk of relapse.
Many addicts find the combination of these effects simply intolerable, especially when the discomfort persists for many months, sometimes years after drug use is discontinued. 
Successful treatment of methamphetamine addiction will require addicts achieving sufficient skills and avoidance strategies to maintain control over their behaviour and sustain sobriety in the face of these interrelated symptoms.
Drug withdrawal is always the exact mirror image of the drugs’ effects; methamphetamine raises blood pressure and heart rate during use, so during withdrawal, both will drop.  Methamphetamine keeps the body stimulated, awake and alert during use, during withdrawal the user sleeps for long periods, has very little energy and feels dull and lifeless.

Craving
Methamphetamine causes intense biological cravings that could be compared to those experienced through prolonged starvation, leading to hunger, or prolonged dehydration, leading to thirst.  In other words, being without food or water for long periods of time would lead to intense hunger and thirst, biological cravings  that are essential to survival, as without them, we would simply die of dehydration or starvation.
One of the key contributors to relapse is the combination of craving and environmental cues.  During drug withdrawal, craving manifests as boredom, restlessness, irritability, with higher levels of craving causing anger, anxiety, frustration, depression and mood swings, all while the user may be confronted with overwhelming temptation of drug use through associates, relationships, ease of availability, old habits and ‘haunts’.
There is a strong relationship between craving, environmental cues and relapse to drug use.  It is often the combination of these factors that leads to loss of control, resulting in drug use.
As an example, think of person who has been without fluids for three days, while suffering intense cravings of thirst, but without money to buy a drink, enters a corner store finding themselves surrounded by chillers full of refreshing drinks.  The craving (of thirst) is intensified in those surroundings, leading to loss of control, and a drink is obtained by what ever means is deemed necessary to the subject at the time.  The craving is satisfied, in an attempt to relieve the discomfort from the withdrawal of fluids.
So it is, that justification is found as a result of the intense biological drives, often regardless of the consequences or implications, which is what lies at the heart of denial; i.e. the need to satisfy cravings overrides any consequences or implications of the actions required to achieve this. 
Now replace thirst with meth cravings, replace the corner store full of drinks with a group of friends smoking methamphetamine, and you begin to understand the immense hold the drug can have over the user through cravings and impact of environmental cues.
Successful treatment models are based around recognising craving and what causes it, and then implementing strategies to manage it without resorting to drug use.
To this end valid treatment plans for methamphetamine addicts are based on careful evaluation and identification of the addicts’ four causes of craving:

Successful treatment relies on the management of craving through avoidance strategies and developing ‘tools’, all of which will reduce the likelihood of relapse due to symptoms associated with cessation.
The addict is unlikely to ever return to the original state of brain function they enjoyed prior to methamphetamine use.  Over time (several months to years) receptor function begins to return and dopamine production improves, but nerve damage is permanent and will leave lasting implications.  The addict can however learn to manage and compensate for this, gradually returning to a life where pleasure can be experienced to a degree where continued sobriety is achievable. 

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References:
Dr Alex Stalcup M.D. et.all
New Leaf Treatment Center
California
United States