Spike only knew Angelus for eighteen years as a vampire before the man got souled, and eighteen years seems like a really short time for vampires -- like a couple of weeks for humans. And yet it's during these eighteen years that they've had to have had the 'except that one--' (time, night, decade?) of theirs. And that it probably would have required all the stuff that happened in the following century for them to really grow into their feelings after this 'one night stand'. For Angel to genuinely become a better man despite the curse, and for Spike to stop thinking of Angel as his 'Yoda'.
It makes sense to me. Spike was a romantic, gentle poet-boy as a human, while Angel self-confessedly was already a mean bastard when he was still alive. And I understand that in the fandom majority's opinion Spike's 'redemption' is seen as somehow cheaper than Angel's, which I can't subscribe to at all. Gentle young boy gets made into a monster and then eventually goes to get his own soul back for love vs. a drunkard lout who killed his own family getting souled by a gypsy whammy because he gnawed on the neck of the wrong person? The math doesn't seem that complicated to me. But it's fitting; two souled vampires who are still each other's polar opposites. Even if, eventually, they would have gotten together, the snark would never end.
And I like both Angel and Spike with Buffy. I think they both would have needed her, as some kind of a phase... a conduit, more than she needed them. And somehow, watching the first season now, I see Angel initially getting attracted to the Slayer as some kind of a subconscious reaction to his knowledge about Spike's fixation with them. And that it was after Spike's appearance to Sunnydale that he first had sex with the Slayer, knowing what it would do to him. I also see Buffy as the factor that freed both of them from the physical and emotional bonds to their sires, Darla and Drusilla, allowing them to move on.
So, yeah. Maybe not what Whedon was thinking about, but I like it that I can see this kind of a story in the background.