Revenge
In a 2004 issue of Science, Swiss researchers scanned the brains of several subjects who had been intentionally "wronged" in an experiment. They were then given a chance to get even with the person they thought wronged them. As they thought about how they could get revenge, their brain activity showed a rush of rush of activity in the caudate nucleus. This is the part of the brain that processes rewards. This gives some psychological confirmation to the saying Revenge is sweet.