In sooth, I know not why I am so sad:
It wearies me; you say it wearies you;
But how I caught it, found it, or came by it,
What stuff 'tis made of, whereof it is born,
I am to learn;

I've always been attracted to this opening of Merchant of Venice - somehow it speaks to me. It reminds me also of Henry Thoreau's quote:

The mass of men lead lives of quiet desperation.

In Buddhist philosophy it is called Duhkha or dissatisfaction, a condition that exists perennially at the base of every action we make.