Oddly enough, one of the symptoms of both unipolar depression and all phases of bipolar disorder is the idea we aren’t sick.
In depressed phase, it is associated with lowered self-esteem. Together with all other negative thoughts comes the idea that we are just lazy, and we’re just pretending, and we don’t have depression at all. (This is greatly aided by having relatives or friends who provide us with helpful phrases along the lines of “why don’t you just pull yourself by the bootstraps” and “you know, you could just try not to be depressed”.) And so we find ourselves on the floor, curled into fetal position, thinking we’re just imagining all this and in fact don’t have any reason to feel bad, so why don’t we get up and do something. Then we don’t get up and we don’t do something, because depression won’t let us move.