In every DC Comics alternate Earth I can think of, Superman and Lois Lane are together.
Other than "because the writers said so" or "because she occupies the requisite-female-love-interest slot," I mean.
I can see why she'd be attracted to him. He's an exotic alien who just happens to be shaped exactly like a tall, athletic, well-muscled, very handsome Caucasian male of her species who also happens to be blessed with godlike powers beyond mortal men. Add that he's a genuinely good man—kind, gentle, caring, courageous, merciful to his enemies, unshakably moral, the whole bit—and throw in his status as occasional planetary savior and most famous man alive...and it's all going to be a bit much for a girl to take in. On the makes-you-feel-dangerous-but-also-safe scale, he makes Edward Cullen look like a creepy geek in the bushes with a video camera, wifebeater, and lisp.
So why does this alien god love Lois Lane? Why her—why this one woman, especially when he could quite literally have his pick of the entire planet? What is so special about her? What does he love about her? What made him fall in love with her?
It seems like it's got to be more than just that his proximity to her in his life as Clark Kent distinguished her from the rest of the women he encounters/rescues day to day.
I have a couple ideas, but I'm no expert on the material. I've seen all the movies and read a few—but not a lot—of the comics. I'm not particularly bothered about which particular retcon or alternate Earth we're talking about, but I'm hoping for more concrete specifics and depth than the easily googled wiki entries on each character offer.
Extra credit: In the various continuities and incarnations, does she usually fall in love with Clark Kent or Superman first? Or, is it usually simultaneous but for different reasons, and then she's torn until she realizes they're two sides of the same man (isn't there a trope for that)?