Why do some people always seem angry

One could write an entire series of books about why people are so pissed off.

I think ultimately it can be narrowed down to -

Nothing is ever good enough for anyone.
People get tired of having to act a certain way to appease their acquaintances.
No matter how rich or how poor you are, life's bullshit never seems to go away.
Looking back with regrets.

Or another one - even people who seem nice are just trying to get something from you.

The older we get, the more fed up with life's BS we become.