To a degree, it is our egos that get involved. Is it any of your business what other people think of you? Why do their opinions matter in the first place? Why do you care?
For writers or any artists, criticism can feel very personal. I know because I've been one of those overly sensitive artists who has dealt with insults about my writing. The truth is that it gets easier as time goes on. It's just someone's opinion. Not everyone is going to like you. Not everyone will appreciate or understand the message you are attempting to send through your work. It doesn't matter who you are or what you do because criticism is a part of life.
I always feel bad when people tell me they want to start a book, but they seem apprehensive because they lack confidence in their abilities. Sometimes self-criticism can be even more dangerous than when other people are dishing out to us. After all, for their words to affect us, first, we must believe them. So wouldn't that mean that a part of us must carry some self-doubt?
Think about it.