Buoyancy

Buoyancy is actually a good metaphor for mood. The image captures nicely both the sense of being above danger when things are going well - light, far-seeing, able to carry some burden - as well as the opposite mood of being slowed, unable to see beyond the next wave, requiring energy to keep any part of oneself above water.

Our base buoyancy comes on us very early. Genetic, prenatal, and early childhood influences are solidly established long before we have any awareness of the question of mood, disposition. Circumstances and actions or attitudes can change our buoyancy, certainly, which is why the naturally buoyant believe they have done something right in order to be the cheerful people they are. Which irritates the hell out of those who know that isn't true.

It is true enough to matter, however, regardless of whether writers of inspirational books and sayings overestimate it. The hardest parts are the twinned demons of hopelessness and anhedonia - that even if we roused ourselves to squeeze some water out of the vessel another wave would soon come, and that even if we were floating better, there's nowhere we would enjoy going anyway.