Why do people engage in self-pity?

It takes energy to be happy. It takes energy to be unhappy. We only have a limited ration of time and energy. It seems a shame to waste either..

There are quite a few unhappy people here. My personal observation is that many unhappy people are pre-occupied with self-pity. This is certainly not always true, but often enough.

Some quotes on self-pity

“I never saw a wild thing sorry for itself. A small bird will drop frozen dead from a bough without ever having felt sorry for itself.”
― D.H. Lawrence, The Complete Poems

“I don’t want it to end, and so, as every therapist knows, the ego does not want an end to its “problems” because they are part of its identity. If no one will listen to my sad story, I can tell it to myself in my head, over and over, and feel sorry for myself, and so have an identity as someone who is being treated unfairly by life or other people, fate or God. It gives definition to my self-image, makes me into someone, and that is all that matters to the ego.”
― Eckhart Tolle

Assuming the strength to admit it, why do people do it?