Try giving this a read. http://www.amazon.com/Stumbling-Happiness-Daniel-Gilbert/dp/1400077427/ref=pd_bbs_1/002-7500355-3640006?ie=UTF8&s=books&qid=1190729827&sr=8-1
Explains how humans are wired to feel the bad stuff "more" as a sort of a survival instinct. For evolution and survival...