In addition to such fleeting feelings as joy or distress, our mental lives exhibit a longer-term emotional character, which we might call 'mood'. Over a period of several months, one might characteristically exhibit (any one of) depression, irrepressible cheerfulness, or anything in between. One's mood, so understood, may be a kind of disposition to react in certain ways, or to rally variously successful coping skills. It may manifest in one's general attitude towards life, as an optimist or pessimist, for instance.
Mood looms large in our self-narrated life-stories. At least, I find that I tend to classify periods of my life according to the remembered progression of such moods. (For instance, there's a certain period in high school defined more or less by depression. And, at the other extreme, the first couple of months here in Canberra stand out in my memory - as a distinctive "period" - for being so consistently wonderful.) It's an interesting question just how accurate such remembered impressions are, since of course they gloss over a lot of details. We may blot out shorter periods that are inconsistent with the general mood: forgetting the good that occurs during the bad times, and vice versa. Should this lead us to be skeptical of assigning much moral import to these narratives (say as determinant of the welfare value of one's life)? Or is it legitimate -- is the dominating memorial influence of mood a genuine reflection of its dominating importance?
(I guess I should also check whether my introspective claims resonate with others. Perhaps I have an unusual psychology, or otherwise misreport what's really going on.)
Questions of 'overriding weight' aside, it seems clear enough that mood is at least an important feature of our welfare. When people speak of the desirability of happiness, I think they're generally getting at the importance of positive moods (contentment, life satisfaction, etc.) rather than mere fleeting sensations of pleasure. So, in light of this importance, it becomes an important question how our moods are influenced, and especially whether there are positive influences that are within our power to implement.