extreme opposites

The Two Are Opposites

"The killer of generosity is Bitterness."

I read the above quote from Seth Godin. You can find his blog post here: Deconstructing Generosity

It's interesting isn't it. In fact the two are extreme opposites, both generosity and bitterness. One is being selfless and the other being selfish.

One makes you happy and the other makes you unhappy. One adds to your life, and the other takes away from your life.

Why then do we gravitate towards one over the other?

Is it because we selfishly want to be happy?