I think you have it backwards. When people enjoy each other and have a lot in common, they look good together. They are happy with one another, they fall in love. This happiness looks good to you, and the comfortable confidence and love eyes makes you increases that feeling.
I agree with anonymousforever. its the same things as when people who smile a lot seem happier or confident. The smile makes them feel confident and conveys that to everyone else. People who like each other seem to physically "work" together because their body conveys their connection.