The audience picks someone to whom they relate

Story Structure 104: The Juicy Details is pre-Community Dan Harmon's advice on storytelling. (He's talking about it in terms of the five minute pilot format that Channel 101 used.)

The audience is floating freely, like a ghost, until you give them a place to land...If there are choices, the audience picks someone to whom they relate. When in doubt, they follow their pity. Fade in on a raccoon being chased by a bear, we are the raccoon. Fade in on a room full of ambassadors. The President walks in and trips on the carpet. We are the President. When you feel sorry for someone, you're using the same part of your brain you use to identify with them.

For standup, this is probably a big part of why self-effacing jokes work so well as openers and get the audience on your side.

More of Dan's articles at the Channel 101 Wiki.