Most of the time, what you say is truth is actually a matter of perception. If you are pretty strong in a certain area and perceive someone as "stupid" or "not as smart," there is likely someone who views you the same way.
You may have read my posting regarding my son, Nate. It does upset and hurt me. I am poor in that area and he was being honest in his perception of my skills. Do I think I have a deficit in that area, absolutely. YET, there are people who are even lower than me in that area. If I perceive them as "stupid" that is truly not fair - they may view me as good in that area although others perceive me as poor. This is why I believe it is perception and not necessarily truth. My son is very gifted in the nonverbal/visual spatial doman but there are people even more gifted than him. If they call him stupid, that again, is a matter of perception.
I don't believe it is a matter of ego for some teachers as everyone is so different. You can't really generalize, although that is often human nature to do so. In addition, people have their own areas of knowledge and sometimes you may have a greater area of knowledge in that area. Yet, there are some kids who are on a more basic level so the way the teachers taught that information may have been at their level. Again, saying it is about ego is merely a matter of perception that has been overgeneralized across a specific group of people whether the group is NT's or Teachers. That is the same as people generalizing and coming up with assumptions about people of a particular race, gender or individuals on the spectrum. Does that make sense.
Why do I think people need to learn how to "tame" that criticism. Sure, you may hurt that person or make them ticked off, but it can also hinder your ability to keep a stable job and network. That is what it boils down to. If you don't care about that, I believe it ends up "hurting" you as for finding a stable job thus having money to live - unless you work at home in a job that requires very little interaction with others (that is so hard to find).