I have a couple of ideas that haven't been addressed yet. Yay!
First off: with regard to the monster/man thing, which has been a part of spike's character since day one, (there was enough of the man left in him to try to save his mum, for instance, and then to dispatch her rather than allow her to remain a monster; the judge, too, remarks upon the humanity yet clinging to both Spike and Dru) I think he is drawn to Buffy because the same war rages in Buffy, whether or not she wants to admit it. Buffy is the slayer. She's a unique slayer, with her friends and her family. Those make her a more human slayer than any to have come before her. But a slayer is still part demon, as she shadow men explained. A slayer is still a killer. A slayer is still a creature of the night. Buffy is not all sweetest and light. She gets off on killing, just like Spike (it seemed like poor Riley only ever got any action after Buffy got all riled up in a good fight). She's the slayer version of "fists and fangs," too, preferring leave the rules to the council and the research to Giles and to let instinct guide her. There is a part of her that only feels alive when she kills. There is a part of her that finds herself superior to her friends and family because she is the slayer. She is drawn to vampire men - to monsters - because she needs someone who can reconcile himself to the monster in her. That's why Riley couldn't cut it; that's why Angel and Spike, who were both torn between monster and man, could. Spike and Buffy wanted the rush of fighting one another before they even knew each other; they grew to respect one another as enemies; they slowly became trusted allies and friends; then, they become lovers; and then finally, they fell in love. They did all of this not by chance, but because they kindred spirits. They were, in many ways, mirror versions of one another: with Buffy wearing her "just a girl" costume to hide her dark side from the daytime world and with Spike wearing his "big bad" costume to hide his human side from the unwaking one. Together, they both dropped much of that artifice and approached a middle ground that was more representative of who they *really* were, not just who the world expected them to be.
That's what I think they see in each other. That's what I think they get from one another. And yes, I do think it cuts both ways. At the end of season 5, Spike said he knew he was a monster, but Buffy treated him like a man. That was why he was willing to fight and die for Buffy, for her cause, and to protect those she loved. Buffy saw the man inside the monster for a reason: because there was a monster in Buffy, too. That's who Spike studied and fought at first, too: the Slayer. He even calls her "Slayer." The monsters in Buffy and Spike drew them together initially; the humans in them always stopped the monsters' hands at the last minute and fell for one another over time.
Then, there's my old stand-by of them growing together -- of them seeing the worst in one another in season 6 -- and coming out stronger for the experience, making it a real, complex, and tested sort of love rather than a hearts-and-flowers, first love perfection goggles sort of love. I'll let Spike tell you, himself, why he loves Buffy. He was quite specific about it. From "Touched."
"You listen to me. I've been alive a bit longer than you and
dead a lot longer than that. I've seen things you couldn't
imagine and done things I'd prefer you didn't. I don't
exactly have a reputation for being a thinker. I follow my
blood which doesn't exactly rush in the direction of my
brain so I make a lot of mistakes. A lot of wrong bloody
calls. A hundred plus years and there's only one thing I've
ever been sure of. You.
"Hey, look at me. I'm not asking you for anything. When I
say I love you, it's not because I want you, or because I
can't have you. It has nothing to do with me. I love what
you are, what you do, how you try. I've seen your
kindness and your strength. I've seen the best and the
worst of you and I understand with perfect clarity exactly
what you are. You are a hell of a woman."