My most recent encounter with unrequited affection has made me think harder about the rationality of partiality. There are many forms that partiality can take (as between friends and family) but i'm most interested in the interest expressed in romantic relationships. The most basic issue is this: how are our partial relationships compatible with our moral commitment to viewing human beings impartially? I haven't come up with anything, save that my own reasons for thinking such partiality might be valuable are sorely lacking - actually, it's a bit worse than that: they're probably inconsistent.
Of course, it'd be a real load off if there were no reason to treat people partially.