This may or may not be TMI, but at least in my case, it was partially just because I have a better relationship with my dad than my mom--and, in particular, River+Rory sort of feels very similar to that onscreen for me. Rory has always been a character I understood better than Amy, for multiple reasons, and River as well.

And since I don't quite have your guys to write in order to explore things, I write what I know, which means I've been writing Rory+River XD