Are we talking about smoking weed or just regular cigs?
If you're talking about cigs then I think it depends on the age, kids may be pressured into it or do it to look cool or whatever. I admit that the first time I smoked when I was like 11 it was because my friends did it and I thought it looked cool, but I hated the taste at the time and never touched a cig again for a while. Then once while drinking and my friends went out for a smoke I was curious "Why do all my friends still smoke?" so I tried it again and slowly got into it.