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.