Why Do Parents Want To Live Vicariously Through Their Children?

I've never understood why some parents want their kid to do a certain thing with their lives. Like dads' who want their sons to be football players, and live vicariously through them. It's controlling and more than that, it's weird. This is what I have to say to those types of parents: YOU ARE NOT YOU'RE SON OR DAUGHTER, AND NOBODY THINKS YOU'RE COOL IF YOU'RE SON OR DAUGHTER CAN DO SOMETHING WELL!