I have some unhappy moments at work, why? I think the whole problem lies in expectation vs reality.

You had expectation + other people ignited your expectations, but reality shows a different path.

It’s like the difference between some words and a kept promise. The first ones aren’t real, the second is.
And in all of this there’s no culprit outside ego.

If you didn’t have the intelligence to understand that your expectations won’t match reality, then it’s your problem to solve.
But then again it’s also ok to change if a work can’t fulfill your needs, in this case an emotional and mastery need.

A need to do more, to help people.
When a work doesn’t suits us anymore we feel helpless, caged, inside a system we aren’t able to change.