Why practice courtesy?

Courtesy is the lubricant of daily life and conversation. It's a set of general moral guidelines that allow strangers to get along easily without requiring extended periods of getting to know each other.

Hello, excuse me, please, thank you; and so on. The handful of general courtesies is all that is required to breeze through normal interactions with your bus driver, your store clerk, the guy you accidentally bumped into on the elevator and a hundred other people who you interact with on a daily basis.

The alternative is to simply be miserable all of the time. The person you hurt the most by being discourteous, is yourself. Walk around being rude and obnoxious and you will get bad service, you will get pushed back, you will get treated as bad as you treat everyone else.

Laugh and the world laughs with you, cry and you cry alone. This dynamic is just as relevant to courteous behavior. How you treat other people will effect how you are treated.

What you should be asking is why wouldn't you be courteous? What exactly is the alternative? Be a jerk and get treated like one in return? all the time? There's simply no point.