Not only does mood affect what you think about, it affects the very process of thinking. This revelation has contributed to the most remarkable shift in thinking about sadness to date: that being down is not only an intrinsic part of being human, but that it can actually be beneficial. […]
When you're happy your brain is more likely subconsciously to base decisions on experience and knowledge you've already accumulated, such as stereotypes. When you're sad, you pay more attention to new information in the outside world - a style of thinking psychologists refer to as "accommodative".