Why do people lose their coolness behind steering wheel?

people can wait in waiting line at grocery store no problem but they can't do the same on the road. I've been in my friend's car and he yells and yells at grannies. He is not a bad kid. I see ton of people muttering in vehemence even when the other drivers see their mouth going foul. I learned to relax because I saw what people do on the road. why do people do that?