[FictionJunction]

Originally Posted by Shashakiro

Basic instincts never forsee consequences far down the road (like lung cancer from smoking, or STD's/unwanted pregnancy from sex). Forseeing long-term consequences is something very mental, and not at all instinctual.

If anything, I'd say that people not smoking is a weakening of basic instincts in favor of actual mental activity, because I think most people who don't smoke do so because they can forsee the negative consequences of smoking in the future.

well, I may have emphasized a little too much on 'instinct.' What you say is true. But still, that woulnd't answer why we would do things that harm us.
In this case, analyzing and the basic principles of isntinct may stand on the ground. While we wouldn't hurt ourselves directly through instinct, it does seem a bit stupid to do so by will.
Although, I believe that I have answered my own query with my first post. We do these things because we enjoy doing them. However, despite us enjoying whatever we take part in.. the fact that we chose to do the action is enough justification.
Ohmygodhumansarestupid.