It's funny too, because I could make up a long list of things Buffy and Spike have in common as well - it's as if he's an expression of her impulsive side, her passion, her thrill in fighting, her zest for life... a lot of things she's eventually learned to put aside in order to be "the hero," just as Angel has learned to do. I think Buffy looks up to Angel in a way, almost in the same way he looks up to her - their whole heroism gig is self-reinforcing. That's why I think their whole schtick of "we can never be together!" plays so angsty/funny sometimes, because it's built on their mutual understanding of Things We Must Sacrifice For the Greater Good.

But then there's Spike, who's just out there winging it, just as Buffy is wont to do when she acts from her true nature... and funnily enough, I think the reason he has such good chemistry with Buffy is the same one that explains why he has such good chemistry with Angel too...