I do think that a lot can affect one's baseline though: health, drug dependency, stress from life, emotional baggage, etc.
And, I think that happiness can be achieved through practice (gratitude, letting go, etc..).
But yeah, that baseline - life gives some people a below neutral baseline.
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.