I used to abuse drugs when I was in high school and college. It started as a mind altering experience in an rural area where it was easier to get pot than beer. I continued with it into college where it was a "everyone's doing it" kind of thing. I actually enjoyed a beer buzz over pot. Pot made me withdrawl and paranoid as well as angel dust and opium. I did uppers and downers a few times. Cocaine did nothing for me. Angel dust was the worst crap I ever did...hated it.
As to why they do it? Some people it makes them feel good. Some calms them down. Some it gives them energy. Some it makes them overcome fears such as speaking to the oppozite sex
Hope thats what your looking for. I havn't done any drugs in 20 years.