I've got free will, don't I?
Deep fried food anyone? Steak with Bernaise sauce (otherwise known as "heart attack sauce") for the gourmets among us? We don't feel the damage they do to our arteries right away, but eventually...
But what occurs to me that's really puzzling is, why don't I eat more of the stuff that is good for me, and I like?
I like fruit, especially citrus fruits, and I genuinely like flavored yogurt. They're healthy, non-fattening, a convenient breakfast meal for someone like myself who doesn't eat breakfast - and when I buy them, they mostly sit around until they spoil.
It's weird, I take vitamins and willingly exercize regularly but can't seem to get in the habit of eating things I like and are good for me.
It's been noted that drug highs, from either mild and relatively harmless ones like pot, or stronger, more dangerous ones like heroin, are really pleasurable at the beginning but as the user becomes habituated lose a lot of the pleasure and become, a habit. So why keep on doing it? I dunno.
And by the way, one of the more astute comments about drinking and pot smoking was made by the late SF author Mack Reynolds. Without doubt pot is less harmful physically than alcohol. But the advantage of alcohol is, that it lets you know it's a poison. You can't get up morning after and not know what you're doing to yourself. But the most conspicuous symptom of pot abuse is a pronounced lack of ambition.