Impermanence.

The source of unhappiness is not the impermanence itself, which is a condition of life to be recognized, but a poignant desire for that which is impermanent to endure beond its time. This includes attachments to people, things, and circumstances that will inevitably pass away, and failure to recognize that many sources of satisfaction in life are temporary and by the conditions of their existence will pass away. When we accept impermanence, "We see the truth of change. We begin to understand how fragile life is and how, most surely, we will lose everything that is dear to us. At some point, in some way, we ask ourselves . . . 'Is there some way I can do this life with my eyes open and my heart open and still love it? Is there a way not to suffer?' The pain of that question calls us to attention, just as it did the Buddha. And out of that attention, the intention to be free is born." (Boorstein, 27)