This is probably my favorite link post that's appeared on LW thus far. I'm kinda disappointed more people haven't checked it out and voted it upward.

The useful & novel aspect of this seems to be the part about the "transition probability matrix." For a long time I've thought of my emotions as like a Markov chain; it never occurred to me that one could empirically find transition probabilities. This implies that the way to avoid depression is to aim for the "bridge" emotions (like "relief" or "hope") to make negative states easier to escape.