To a degree, it is our egos that get involved. After all, is it really any of your business what other people think of you and regardless, why do their opinions matter in the first place? Why do we care?
For writers (or really, any artists) criticism can be taken very personally. 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 certainly gets easier as time goes on and you learn to not take it as personally because, at the end of the day, it's just someone's opinion. Not everyone is going to like you. Not everyone is going to appreciate or understand the message you are attempting to send through your work. It doesn't matter who you are or what you do, criticism is just a part of life.
I always feel bad when people tell me they want to start a book but they seem apprehensive because they're already doubting themselves and their abilities. Sometimes it is that self-criticism that is even more dangerous than the kind other people dish out to us. After all, in order 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.