Why are people chastised for "getting defensive" when they receive rude, non-constructive criticism or remarks?

You seem to be heavily implying that this is a guy in this scenario. I have to fully admit that I'd seriously tease a guy who got upset over being called out rudely. Why? Well because guys hassle each other a lot, and as a guy you need to be able to take it to look "manly". This may seem harsh, but it's just how guys are supposed to be. Maybe in this case it'd be better to reply, "Why? You need the bacon instead, chunks?" aka A mocking response for mocking criticism.