People only blush when they feel humiliation after pride.
Blushing means the involuntary reddening of your cheeks.
People only blush when they feel a particular combination of two emotions - only when humiliation quickly follows pride. You will blush, for example, if you walk into a room of people who look at you and then laugh at a big stain on your crotch. You feel pride when you first walk in the room because everybody is looking at you. You then feel humiliation when you realize that you have a stained crotch.
Blushing makes others feel compassion.
Blushing and crying are the only expressions that you cannot express voluntarily. They are also the only expressions that cause others to feel compassion. People who see you blush or cry will want to help you.
Blushing makes others feel compassion instead of humor. The laughing people felt humor when they saw your stained crotch. They would feel compassion when they looked up and saw you blushing.
Blushing is not evidence of a problem.
Blushing is a normal response in situations where you are learning by trial-and-error. Success makes you feel pride. Failure makes you feel humiliation. Blushing makes others feel compassion so they don't laugh at you and discourage you from trying again.
Public speaking is a good example. You feel pride with everyone looking at you at the podium, but then feel humiliation when everyone hears you stumble. You blush. The audience feels compassion and doesn't laugh. You keep going and don't stumble the next time.
Young people are more prone to blushing.
Young people are more likely to blush because they are more likely to be learning - the only situation that routinely produces the required combination of emotions. Older people do make mistakes and feel humiliation. But they rarely blush because they rarely feel pride just before their mistakes. They usually learned what they are doing and felt the associated pride long ago.
Young women are more prone to blushing around older men. Women have the added issue of infatuation, which is usually stronger for older men. Consequently, young women feel pride and strong infatuation around older men. This heady combination makes young women more prone to stumble verbally and then feel humiliation and blush.
For more about emotions, visit: Happiness Dissected