For me I love adorkable types and Krom fits this more than 100%! Plus, I have a thing for blue hair (yes I'm shallow). I also can't deny that the idea of a Lucina who is Fauder's grandchild is epic, to me, and that the MU seems to have a legit reason to be with Krom given that he's in her thoughts when she awakens in that field.
All in all I can't provide a single 'unique' reason, but I haven't found any of the other guys in-game to be all that interesting, either. I'm sure the characters have interesting back stories, but that doesn't change the fact that I'm no where near interested in them - where as pairing with Krom does interest me. Therefore MUxKrom! :'D
Plus.. Marth's heir! Who doesn't want to associate with Marth! It might get you a placement in SSB!! (<--- Seriously this is a joke)