I reckon sometimes it starts with dumbassery but then you're just stuck. i.e., you start smoking because you're a dumbass, then later you may no longer be a dumbass, but you still smoke.
I've known a few "serious" smokers who've successfully quit, but I emphasize the "few." I've known more who can't seem to kick it. Some go for a long time and think they've got it whipped, but then start again.