Tbh I think I can understand why people do it. For some using various substances are a quick and simple way to relax/numb unwanted emotions/etc.. Some start because of peer pressure. For others it's just learned behaviour (you're more likely to become a smoker if your parents are).

Personally, I've never as much as tried cigarettes because it just never appealed to me and I had mostly non-smoking friends and family. But I definitely have other bad habits I shouldn't have gotten into and have trouble getting rid of, so I get the feeling.