Probably because while you are doing it, it sucks. The payoff comes later. It really is the only way to leave it behind.

You know, when a child is growing up they need to be able to process events that happen to them and if they don't get to do this, they store it. These events and memories don't go anywhere. Feelings need to be processed. This is what makes our bodies function correctly. You have emotions, they need to be processed. What you are doing in therapy is this exactly but you are doing it with stored feelings.