The brain works in circadian rhythms and we slip into melancholy without always understanding why. What we know from experience is that there will be a return to form if we don't despair and we are patient. In time, the tide will return and replenish our mood with newly gained perspective; or so we hope. Our wallowing in self pity is sometimes tempting but then it only counteracts our ability to rebound.
Our lives are far from perfect and we know it and it is at times that very thing which spirals us into moroseness. But then we remember everything we should be thankful for and we come back to a place of normalcy; one in which we know there is no perfection but instead a calm resignation about the fact that we are human and subject to frailty.
Aging hopefully brings us that perspective which aids in comprehending our own patterns and gently pulls us away from the precipice.