Good question.
While still very young my mother contracted lung cancer. As such I immediately developed a hatred of tobacco. That explains that one.
While still young I watched my father be an alcoholic. My only exposure was to take a sip here and there to see what it tasted like. I found that beer was disgusting, but wine, champagne, and hard alcohol tasted very good. Even so, I wanted nothing to do with them after seeing how it had affected my father. I asked many people about drinking and by the time I was 17 had a pretty good understanding. It seemed it was about peer pressure, it was about doing what kids weren't supposed to (especially when it's just an American stigma), it was about relaxing and throwing off artificiality which rules the lives of children (even though they inwardly desire to abolish the cliques and seperatist mentality), it was about being happy and forgetting the bad things, if only for a little while.
I had a bad experience when I was 17 and couldn't bring myself out of it. My friends older sister offered me a bottle of sour whiskey so I could just get smashed and finally sleep for a full night. I did. It worked. For at least the time I was drunk and the time I was unconcious

I didn't have any problems. Fortunately I don't suffer hangovers so I didn't have to pay for the escape either. I decided then that although I wouldn't ever become a casual drinker, alcohol was one method of escaping reality that I could use when absolutely necessary. I have continued in this manner of thinking. I very seldom drink, but when I want a night of throwing everything out the window, I drink like a pro. I also LOVE the taste of wines and often have a glass with dinner or desert, but no more than just a glass. At that level, alcohol actually exhibits positive medical properties.
As for drugs, the reasons to do them are the same as the reasons to drink alcohol. However, unlike alcohol, the negative effects are far more dangerous and unpredictable. People will argue that driving drunk is more dangerous than driving with a joint, and they're correct. However, driving drunk is a choice that you can avoid. You CAN drink seldom and responsibly. You never hear of anyone drinking laced scotch (except Rohypnol and other date rape drugs which is a different subject entirely) and dying from it. Long term alcohol abuse is certainly damaging, but a single use of E or LSD or laced pot CAN kill, or severely damage. As long as alcohol is kept very seldom and moderate you won't see the serious damage everyone screams about.
Also, I've seen too many people (and known even more who've seen too many people) lose their personalities to drugs, especially marijuana. People who only smoke once or twice a week who over the course of a few months turn into some complete freak, incapable of functioning in a non-druggie environment. So many brilliant lives that can't form a cohesive thought any more. I have, in all fairness, known some people who are VERY RARE users who can go their entire lives without negative effect.
Anyway, that's my observations...everything is about escape, fitting in, feeling a part of something...at any cost.