good sex, but crappy relationship.
That adage, re-quoted by FallingStarr, describes my wife or 32 years and I. I am outgoing, she is quiet. I am a hellraiser (or was when younger), she was meek. I can talk to any stranger and carry on a conversation, she will give one word answers to strangers or say nothing at all. Switch us around, and we'd still be the same couple.
Most people like people who are like them Throw in some interesting differences, and there it is!