Why do we do.....
Smoking - because it looks cool and gives you a nice little headrush (until you do it too much and don't get the rush any more).
Obesity - because cake is delicious, as is fatty pork, bacon sandwiches, huge steaks, chips, mmmmm.
Alcohol - because getting just a little bit drunk is awesome and makes everything else better - food, sex, smoking, talking, listening to music.
Lack of activity - because once you've spent long enough doing the first three you can't really do much anyway.
The reason I've cut back or cut out all of the above is because too much of a good thing takes away from how good it is. Now when I have a drink, I appreciate it so much more, and a big fatty takeaway, now it's so rare, is much more enjoyable.