people's bodies just get used to certain drugs and once a person gets a tolerance, they can handle more and more.
i'm guessing there's not really much to explain with the chemistry in the brain cause with other drugs that effect the brain differently people also build up huge tolerances with.... obviously some one could explain what happens in the brain and why people will need more and more opiates, but dramatic tolerance issues occurs with a lot of different drugs.
i personally have problems taking even starter doses of benzos and stuff like flexeral doctors have given me.. like i only needed 1/8th of a starter dose of flexeral. the first time i tried half of what the doctor prescribed and a beer, and it was way too much for me. everyone is different.
anyways, if you ever go back to these drugs, which hopefully you don't, just make sure you realize you won't need nearly as much as when you were using.. probably obvious to some people, but i don't know how into harm reduction the OP is... you may even find yourself with the same tolerance as your first time using if you have taken enough time off.