She believed in him yes...but why do you think it was so easy for her to let him go? To let him die? Why didn't she argue with him when he denied her 'i love you'? So many things lead me to believe she loved him yet so many other things don't. It's difficult for me. I want to believe she loved him I just don't know if she had the capacity. Sure, she really cared about him and she trusted him and consitered him a dear friend at that point...I just don't know.
Too bad she doesn't know he's alive...(*sighs)
Ah, yes, the old, "No you don't, but thanks for saying it" thing. Now, why WOULD someone who, when the time came, when there was no other choice than self sacrifice, not jeopardize everything that they had fought for, suffered for, in some cases even died for, accept the fact that there are times when, as Mr Spock so eloquently put it, "The needs of the many outweigh the needs of the few. Or the one,"? Maybe because she had had to make that same decision just two years earlier.
Why didn't she at least stay there, when Sunnydale was falling on top of her, and debate the point with him? ("But I do, Spike, I REALLY DO. That clown Angel was just a school girl crush. You're the one, you're the ONLY one. We belong together. Our fates are intertwined. I have a job to do, building and organizing this army of new Slayers that I had Willow create through the power of my magical Slayer Scythe. WE have a job to do. The world needs you. I need you. Please, oh, please...") She barely made it out as it was, and she HAD to make it out.
Remember what Spike had told her? "You're THE ONE, Buffy...) Well, she WAS the one. The one who has caused all of those girls to be Called. It was her job to find them, organize them, train them, and lead them. If she didn't do it, who would? Faith? Giles? Willow?
How about Andrew?
The sacrifice had to be made, Spike was the one who had to make it, and it had to be made RIGHT NOW. So she left him down there in that hole to do what Angel has never been able to do. To redeem himself. His sacrifice in that moment tilted the scales far over into his favor in those last few moments, because, in voluntarily making that sacrifice, he saved every living thing on the entire planet.
Oh, and, by the way, Spike died knowing that Buffy loved him. That's what he was laughing about.
And Buffy knew that Spike's actions there had proven to the world, undeniably, that a monster could become a Champion, and a Champion deserved heaven for his labors. That's what she was smiling about.