I was once in a horrible mood... I felt really guilty/regretful about something I'd done earlier, and felt terrible. Then I was distracted for about half an hour by math homework, and when I was walking outside a few minutes later I caught myself whistling. I was like "Whoa, self! You're supposed to be upset right now!" and almost descended back into the pit of despair, but then I stopped midstride and said "Wait a sec. Why would I want to be upset?" and so I didn't. I kept whistling and had a great day.