What went wrong? When things don't go to plan, we want to know why, and this applies to moods just as much as it does to missions. When you're down, it's understandable to believe that there must be a cause or reason (even though there isn't always one - sometimes low mood can just strike of its own accord).
But there's a different sort of question which we don't ask nearly enough: what went right? If you have a day on which you feel better than usual, it really makes sense to try and pin down why. This time you may be more likely to succeed. Good moods are often inspired by something, or a collection of things. Feeling good isn't simply the absence of bad - it actually needs to be the presence of good. Of course it helps to know what might take you down so you can avoid it, but it's just as useful to know what may take you up. So you can do more of it.