A bartender needs a good sense of humour. Your customers will often need cheering up, or have funny stories at which you have to laugh. A good trait in a friend is a sense of humour; and a bartender's number one job is to be a friend to his patrons.
You should also always have a good joke or two. Just remember, not all your patrons want to hear about when your fishing buddy got his balls caught in a window sneaking out of a college dorm after a one-night stand; as Cunningham put it, always have a joke you can tell your mother.