Sincerity is communicated more by the right moment than by the words. I think very beautiful women may have more difficulty accepting a compliment, because they hear them all the time from all sorts of people who are trying to sleep with them.

Guys don't always process "the sure thing" in the depths of their minds. An insincere guy is receiving instructions from his penis to issue compliments at random, in a desperate effort to get laid. It's a habit for the habitrail set; chasing after the cheese.