One of Robert Cialdini’s seven principles of Influence. If we like someone we are more likely to be influenced by them. This could be filed under the heading of Stating the Bleeding Obvious, except that – as with many of our systematic biases – knowing about it is no defence.
Cialdini lists several ways in which liking can be engineered, including similarity (people like us), compliments, contact, and cooperation. It also helps if you’re good looking. Damn.