Addicted to porn?

What makes you feel like you have to do things over and over? Sometimes a song got stuck in your head. You are listening to it over and over, repeating the same song for some days. After a certain time span you will stop listening to the song. It becomes boring what has been so exciting at the beginning. In contrast to that there are things that are establishing a habit. Pornography and drugs are such things, while it is not completely clear why we differ between pornography and drugs. In our brain the same reaction takes place when we consume drugs or watch porn routinely.

Why is pornography and ego-sex so pleasing, at least for a short time?