Well, generally speaking, it's a feeling of character flaw. All of those things that someone else is are things that you'd sometimes like to be. Kind of like in "Fight Club". If you're a manly dude, low on emotion, tough guy, etc, you would most likely be happier with someone who is emotional, girly, nice...because it rounds you out.
You won't want someone just like yourself because you are already you, and you need those missing ingredients to make yourself 'whole'. Much like Aristophanes tale in the Symposium. We are looking for our other half, which cannot be found in something exactly like ourselves. The whole 'opposites attract' thing is completely true.
Look at it like a diagram of a person. On the outside is you, whatever it is that makes you you. Personality, looks, etc. Then under that is another layer completely opposite of you. Then under that is your core, which is the same as the outside. Outside and core is you, but there's that middles piece that you really are not, but sometimes need to be. The only way to be that and fill that middle section is with someone opposing your core. Meh...You get that idea.
I decided to go middle ground. Shortly epic post. You're intelligent enough to not have to explain myself more, but if you have questions about these thoughts, that'd be easier.