No. No. No. No. No. That's a lie perpetuated by some of Spike's fans. Spike never changes, ever. He's a character with virtually no character development throughout the series. He was obsessed with his mother, then with Cecily, then with Drusilla and finally with Buffy. Everything he does is a byproduct of that obsession. Everything. The only factor that changes are women.
Spike may be passionnate and act majorly depending on the women in his life, but that doesn't mean he never changes. Being dependent (that's how I interpret the way he gravitates around the women he loves) is only part of his personnality, not all of it. The first time Spike did something good, it wasn't because he loved Buffy. It was becuase he liked the earth as it was, with humans being like "Happy Meals walking around".
After he came back in Sunnydale in S4, he started helping the scooby-gang, but not by choice. Thenn, he got acclimated to them and started helping because he wanted to. But it took him a while before he realized that he loved Buffy. And if I remember correctly, he wasn't so thrilled about it at first... If you don,t want to be in love with somebody, why would you help them, except if it helps you too at the same time? Spike kept helping the gang because it served him to a certain point (i.e.: doing good didn't give him a headache).
Yes, eventually he kept on helping the gang because he loved Buffy. But he also loved Dawn like a little sister, he liked Willow and Tara, he even liked Xander to a certain extend.
He didn't help only because it was what Buffy wanted. There's way more to that and saying that he never changes and only goes the way his obsession does is you being a very bad judge of character, imo.
Again, voluntary brainwashing is still brainwashing and a decision to make a change is not a change. You know what would be a real change? Spike becoming good without a chip in his head, without Buffy and without William's soul. That would be a real change, but such a transgression is beyond the grasp of a vampire. Unlike humans, they are incapable of it.
Vampires have been depicted as monsters. They're supposed to be the ultimate paradoxe, a monster that looks like a human. But they used to be human, they used to have a choice to be either good or evil. So what, because they've been turned, they can't make a choice anymore? Or is it just that the choice of being evil is way easier to follow when you don't have a soul? I vote for the second option. It's not because whe haven't seen a vampire not be good without a soul that it couldn't be possible, especially in a universe where all other possibilities have been showed at one point or another. Spike and Angel went through different path, both leading to the same end, their soul restored. But that doesn't mean that if they never got their soul back, they wouldn't have changed in the same way, just on a longer run... You can't get proof of how vampires can turn out when given the real choice to be either good or evil with the same path and consequences.
No. Angel started fighting evil as soon as two years after his ensouling and became a regular champion during First World War. It was a result of gypsies' spell, nothing else. He started doing nothing again in the 70s but got back into business because of Whistler's interference.
"Angel became a regular champion during World war 1...."??? Except if there is a comic out there that shows him being a hero, I can only recall one situation where he helped during the war. And it wasn't his choice. He wanted to be left alone, but the "government" wanted him to help and so they made sure he would help. I think I remember them threatening him at one point... Angel was (dare I say it? oh why not) pathetic after he got his soul back. I don't see him helping the Americans during any war, especially when he didn't care about it at all.
He started helping for good when Whistler came to him.
_________________
But this discussion is about Spike right? So here's my thoughts:
Buffy knew what having his soul was doing to Spike. She saw how tortured he was. despite the way they both had threated each other in the previous year, he had became a part of the gang to her. And having his soul back meant for her that he was good.
She believed in him, believed in what he could be. He was an ally and a friend. After all, wasn't he the first one she told about being in Heaven in S6? She obviously liked and trusted him.
And along the way of S7, when she was in need of someone to listen, to have her back, to keep her afloat, he was there. She knew he would always be there.
And of course she loved him. You'd have to be blind (or really don't want to) to not see it. She loved him enough to keep him close and protect him the same way she knew he'd protect her.