I read an interesting article that stated each of us has nearly 60,000 thoughts per waking day, one each second. We mostly think about things we have thoughts about before and many of those thoughts are negative.
Mood and memory are linked.
For whatever reason, bad thoughts seem more powerful than good thoughts. We hash over the bad thoughts and we get in a bad mood. People in bad moods think negative thoughts. We should call it a negativity loop.