I think it comes down to the fact that conflict is more interesting, and a divide like that is a very easy way to have it. It's also a wonderful way to explore the two characters - their morals, their indecision, all of that delicious inner conflict. (Well, in this case, Buffy's morals and Spike's bewilderment and frustration.)

I liked Buffy/Angel well enough on the show, and I felt for them, but I was never very invested - because they seemed more in love with the idea of each other, and while their connection was very deep, it wasn't very identifiable. (And, in the long run, I think she was far better for him than he ever was for her.) I'd be more interested in them now, I think, after both having grown apart from each other, into more mature people. At the time, Angel frankly didn't have as much of a personality.

Epic romance and soul mates and all doesn't really interest me, as much as, well, people, and you get that more in enemy ships too, I think.

My favourite sorts of ships are friends turned enemies - Doctor/Master, yes, and Dumbledore/Grindelwald, and Xavier/Magneto. When there's still a lingering fondness, an old respect, that they can't quite rid themselves of, even as they fight brutally. Of course this will vary with just how different they are, and how mentally stable. (Hi, Master.) And Angel/Spike, sort of! Although that ship had to go through a lot of changes as we learned more about their history.

...That was a bit long, wasn't it. > >