Methamphetamine (thing)

To those with a background in organic Chemistry, the reason for the high addictivity and mental chaos meth brings should be obvious -- Methamphetamine is effectively the Crack Cocaine of Amphetamines.
While Crack cocaine increases the Cocaine's effect by dropping the HCl ion and giving the user the unadulterated drug, fast, Methamphetamine works its magic by adding a Methyl group to the Amphetamine molecule.

Amphetamine

  ___  ___  ___NH2
 /  \\/   \/
||   |    |
 \__//     CH3

C₉H₁₃N

Methamphetamine

  ___  ___  ___NH
 /  \\/   \/   \
||   |    |     CH3
 \__//     CH3

C₁₀H₁₅N

You see, Amphetamines have somewhat of a safety feature -- once you eat too much Adderall, Dexetrin or whatever else your doctor prescribed your kid sister, your brain simply won't uptake any more. Sure, you can eat pills by the handful to stay awake for a few more hours, but the thrill is gone.
Methamphetamine basically says "Fuck you brain, if you desensitize your receptors, I'll just cram it somewhere else, and twice as fast!"

This also eliminates the possibility of switching to a less harmful speed and eventually scaling down to sobriety. After doing Meth for any amount of time, one's brain won't uptake anything "weaker." Amphetamines have no effect, and even MDMA provides little, if any, rush.