I think that most people are at a neutral level of happiness / sadness. Something good happens... it makes the happiness level go up, then come back down to neutral. Something bad happens, it makes the level go negative, then come back up to neutral.

A few people are naturally positive. They're just happy most of the time. Their bounce back state is happy, rather than neutral.

And, a few people are always depressed. A bad experience makes their state of being much, much darker. A good experience may bring them to the neutral line or perhaps slightly above, but they go back to depression as their normal state of being soon enough.

If I were to guess, I would say that early childhood trauma is the greatest contributor. Children deserve better everywhere.