hard on

What makes you super erect with the hardest your cock can get? I noticed since my late teens I can be hard and then sometimes get super hard erection like my cock hurts because it's so hard.

I have heard from a straight young friend that is impossible to him to have a hard on seeing a guy naked or with that guy with a hard on. Nothing moves him. I am aware that if you don't want to make that situation sexual it doesn't happen (the hard on) (talking about myself). The question is...