Curiosity, really.
Even as a little kid I was fascinated with drugs. I remember first hearing the word "cocaine" on the news or something, didn't know what it was obviously but I loved the word! Cocaine! Sounded mysterious and interesting. Later on I started writing stories for the school paper, must've been 8 or 9 or so, wrote one about a coke dealer & got in trouble, lol! Fascination stayed and I vowed that would I ever get the change I would try it.
Many years after that I caught a friend of mine with a piece of rolled up paper. What are you doing man? Ssshh, this is coke, don't tell anyone. OK, I won't tell if you give me some!
Been doing coke now for 16 years, other drugs as well but that's more or less how it all started.