I married Derkeethus with my main character, a Khajiit, because the beastfolk races are my favourites and I really wanted a Khajiit/Argonian romance. Plus, I loved the idea of them falling in love after she rescued him.
My Argonian mage married Onmund, and my Redguard assassin Jenassa, because they both have similar lives and personalities to my characters. My Altmer warrior married Lydia, because I pretty much made him to shun all Altmer stereotypes, so of course he had to marry a Nord! And they just suited each other. Finally, my Dunmer thief will almost certainly marry Marcurio, because he's as snarky and overconfident as she is.
Yeah, I kind of like interspecies pairings...