Why?

Why is it that the more I try to eat less, the more I think about eating (food, what's in the pantry and how can I get more chocolate frosted chocolate doughnuts)?

Why is it that I can look in the full-length mirror at home and decide I don't look quite as bad as I think I look, then go to a store to try something on, look in the full-length mirror and think I look several times worse? (The solution there might be to just avoid mirrors in general.)

Why can I think of something clever to say when the time for that particular clever thought was several days ago?

Why?