It's a very good question. Why do some people become addicted to drugs? I can only answer using my own experiences, and what I have observed being in the company of a whole lot of other addicts. The answer, really, is very simple - I (and I'm guessing most other addicts) no longer wanted to participate in life, in reality, in the world around us. Two things you have to understand about addicts - the first is that their ultimate goal is oblivion. They want to "check out" if you will, divorce themselves from the world around them by activating and abusing the chemicals in their brain that facilitate feeling good, calm, and unafraid. The second, is that most addicts are smart; we are clever people who, unfortunately, have become overly sensitive to the world around us. Try to imagine a brain that works too well, so much so that it becomes a burden, taking in too much information and processing all that information to the Nth degree, focusing, primarily, on any immediate threat to it's survival - past and present. For me, it was a means to forget the past, and not only forget the past, but eliminate the past entirely so my behavior no longer reflected it.
That's sort of a basic answer, and it doesn't even begin to describe the escalation, or a person's "drug of choice," but I believe it begins with the discovery of a means to escape.