Why do people do things that they know are bad for them?

I think it all boils down to comfort - whether its smoking, eating, chewing your nails or whatever, it's a natural instinct during times of stress. Perhaps the most helpful thing for people is to identify WHY they fall back on such things. My mother only really showed me affection through food ("here, have a packet of sweets")- therefore I now stuff my face and equate that with pleasure!

So if you can get it clear in your head how irrational it is to do such things you're half-way there to giving up.