I know a lot of people that have taken or still do take drugs and get intoxicated with alcohol. People get offered a ciggarete or joint at some point.

Situation: A guy is sitting with his "friends" in an alley. At one point, one guy takes out a Marijuana joint. They start passing it around, and when it reaches him, the guy hesitates.
Now, the rest of the gang tease him: "What's the matter? Afraid of the cops?" "Hey, guys, we got a chicken here!" "C'mon, try it, you'll like it!"
The guy hence collapses under the pressure and accepts. He feels good to be part of the gang again.
Problem is, the next time this happens, it'll be easier, and eventually he'll become addicted.

My question is: Why do so many people accept to ruin their only body? It's absolutely silly in my opinion.