What is it and why does it happen?
Let me remind you again what you know.
The reason for disappointment is keeping expectations high.
It goes through your assumptions in the expectation summary. There is an assumed world and a real world.
“High expectation” here varies quite a bit according to your criteria, plus it’s the only factor that changes how much you can contrast with the real world.
Frustration can cause you to have low motivation, and in severe versions, it can cause side effects such as depression.
Unlike disappointment, very few events will happen within your control that will surprise and delight you.
Avoiding this, on the other hand, is pretty easy. Keep your expectations low. And remind yourself of this as often as possible.
In this way, maybe life will come back to you with nice surprises.
At least on the scale of your expectations.