Imagine you get busy because of external deadlines you cannot influence and you need get things done and don't have time to think a second about anything unrelated.
Even if you are in a bad mood and don't want to work you MUST because of the deadlines. Once you are working towards the deadlines, all bad feelings are gone.
Not sure how it works the way around, I am happy, now I make myself busy with random taks?
Edit: Still thinking about your opinion and now I might agree. If you feel good you might have the willpower to start new meaningful things which make you busy again.