Because no matter what you say in life the truth will always be the truth. You know when someone is telling the truth you look in the eyes. I have a tendency to believe people.
If I can get you to laugh with me you like me better which makes you more open to my ideas. And if I can persuade you to laugh at the particular point I make by laughing at it you acknowledge its truth.
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I will make a bargain with the Republicans. If they will stop telling lies about Democrats we will stop telling the truth about them.