Why are most Super Heroes Mentally Unstable?

If Super Heroes were made to have a psychological evaluations I’m positive that most would fail…..

I’m doing research for a book I plan on writing about a superhero who losses it during the course of the book and starts the transition to super villain…...So

What do you think the reason why so many would fail…?

Is it because of the initial trauma most go through when becoming a super hero?

Could it be caused by the stress of living 2 lives?

The stress of what people expect of them?

The stress of the “job”?

Having to be/appear “normal” at times?

Dealing with the psychotic villains on a regular basis?

The constant reminder that their short term work usually ends up being useless in the long term?