Dieting makes you feel deprived. 

Dieting causes resentment.  You feel like you are being told what to do, so you rebel.

Dieting is rigid and unnatural.

Dieting causes you to ignore your body's cues.

Dieting is a compulsive behavior... It leads to perfectionism.

Dieting makes you dependent.  You follow an arbitrary diet devised by someone who does not know you or your body.

Dieting makes food evil.  It leads you to believe that there are good and bad foods.  Most importantly, diets make you believe that YOU are bad if you eat bad foods.

Dieting leads to increased weight gain.  Each time you diet you run the risk of lowering your metabolic rate which makes weight loss even more difficult the next time.  Biology has made our body's very efficient at storing fat and this is what happens when you diet.