As many people stated, you're lucky you didn't get the grip of it. I once had a friend in high-school who tried weed, just because he was curious. He promised me that he won't get addicted, and he did indeed kept his promise. This also depends on the substance you used. Heroin is probably one of the substances with the highest rate of addiction among all.
Well.. that is a wonderful question. You see, each one of us has a unique set of physical and chemical predispositions which when combined form a one of a kind response to drugs, alcohol or any other substance. It's in our genes, our DNA.
Honestly, I think even the talk about this a little off putting. Even just by asking the question, it might give some people the wrong impression that maybe they can try heroin, and not have a problem. I just to want to be clear to everyone that I have known people heroin problems and that was exactly their thought before becoming addict.