I believe the reason to blush is because of embarrassment or stress. It results from overactive sympathetic nervous system that is part of the nervous system. Extreme blushing can be physically and emotionally intolerable. Learn more from http://eredicane.com/blushing.html.
When I was texting one of my guy friends and said I wasn't pretty and he said: who says you aren't a pretty girl? I'll go beat up whoever said that! It was kinda funny but I turned super neon fluorescent bright glow in the dark red