If you don't know why humans blush, don't be embarrassed: just watch this video. You might be able to guess that it's to do with increased blood flow in the face—but even scientists are still a little puzzled by its evolutionary basis.
One point worth noting is the fact that there's evidence knocking around which suggests that blushing makes you more favorable to onlookers. So while it might make you feel hot and flustered, it could be making people around you feel the same—but in a good way. [ASAP Science]