Well, I don't believe Angel did love her from the start, but felt empathy for her. He was already good at that point, he just wasn't doing good actively after a few failed attempts (seen on Angel in flashbacks), so I don't feel that he did change that much for Buffy or due to his love for her. He started to help, and then he fell in love (and like most of us, once we are in love he got all mushy, saw her through rose-tinted glasses and thought he'd always loved her because she represented something so new in his life), but it didn't greatly change him.
After he couldn't get back with Darla, he stopped feeding off humans but he drank human blood all the way through to 2nd season Buffy.
In flashbacks he attempted to help one person, once. He knew that person and all the others were under the influence of a paranoia demon - but when they were paranoid and hanged him, he told the demon to kill them all. That's the only point that I recall that he tried to help someone and it resulted in an ugly mass murder. That was during his brooding phase - which seemed to have lasted for several decades.
He seemed to veer from one mood to another - he also had a manic phase - spending time in Vegas with the Rat Pack and Bugsy Segal. Getting close enough to Sammy Davis that Davis took personal advice from him at least twice. Crashing Elvis's wedding, and meeting with mob bosses. In the 70's. he was looking groovy, and in the 20's he had enough disposable cash for an extremely nice suit, which he wore to rescue a puppy.
I see Angel as desperately trying to fill a hole inside himself, but unable to do so. But I don't see any indication that he even cared about humans in general, or bonded with any human. Even Judy, who he tried to help, who he knew was under the influence of a demon, he left to be devoured by that demon.
It was Buffy who showed him that he could fill that hole by helping others - however I never saw any indication that he went out and helped others on his own. Mostly he was with Buffy, or helping her in some way. He helped the others much like Spike helped them the summer Buffy was dead - but only because of Buffy. It wasn't until he left her that his helping others really blossomed. So - while Buffy did love him even before he loved her - I don't see where he was less selfish by the standard given for a souled vampire. He even resisted breaking up in season three, when Buffy told him that she knew she could never have what she wanted from him.
Angel changed because of Whistler just as much as Buffy. Whistler gave Angel a mission, a focus- to help the slayer- (also there was the whole I'm a demon and I'm not that bad' thing).
I completely agree - Whistler built him up with some tough love - and brought him to the point that he could care.
Spike changed because he wanted to get back with Buffy after the AR. He returned to Sunnydale in S7 for precisely this purpose- 'to be what she deserves'. so what he did was more focused on their 'relationship' and wanting it to continue in one form or another. He would benefits a lot from that so it is more selfish.
His actions in S7 show how selfish.
Plus Angel loves Buffy but leaves her alone, Spike is always around her for whatever reason. Seems more selfish to me.
Yes, he did want the relationship to continue when he went off to get his soul - that's my point. I can't really think of a better proof of love to be the only demon who ever fought for a soul. Once he had the soul, he realized this could never be.
In season 7, he spent about 1/4 of the episodes in the school basement - but he did what he could to help Buffy. He specifically told her, the one time he left the basement, that he did not expect to get back together with her, and he never tried to. Buffy was completely in charge, and he tried to be what she deserved - a strong ally. He offered to leave, he begged her to stake him on more than one occasion , and he fought beside her.
Spike is always around her because he lives in her basement! Angel had his own apartment. But it's Buffy who calls on him to help her - killing demons, training slayers, doing recon. The one thing he does is he never questions her - he is there for Buffy. Angel was much the same - he was there for Buffy. Yes, Angel's personality is less abrasive when he is souled - but if we count in the unsouled period he was far more destructive. There is no way Angelus would adapt to being chipped, nor would he spend a summer keeping Dawn alive and fighting demons. Not good - not bad - just different vamps.
And, as far as leaving her alone goes - once Spike burned alive and came back he never contacted Buffy.
If the roles were swapped in I Will Remember You, would Spike have given up his humanity and his chance to be with Buffy to save the world and prolong her life somewhat? I don't think so personally.
Of course he would - he has no desire to be human, so not much sacrifice there. But he did burn alive to save her from dying as well as to save the world. And he also realized he and Buffy had no future, and he didn't try to get in touch - even in TGIQ he said he said she didn't love him but he was still her friend.
There is no evidence that Spike returned to Sunnydale for that purpose. He never sought Buffy out, he was staying in the school basement and it was Buffy who casually found him. After that he goes to her and to the rest of the scoobies and offers his help. Btw he does it openly, not stalking her in the night, and despite the risk of being staked (by her or someone else of the scoobies). He never asks anything in return and never tries to restore their physical relationship, not even when they sleep together in the same bed. And he ends burning in the Hellmouth to save Buffy and the rest of the world.
This - you said it well. I still don't see where souled Spike is more selfish than souled Angel. They are very different in personality, and Angel/Angelus is definitely a split personality. Spike's demon and soul are more integrated, probably because his soul wasn't an unexpected curse that his demon tried to fight. Spike loved Buffy in a demonic and possessive way before he got his soul - but he also didn't want to see her hurt, to hurt her himself, so he went and got his soul to protect her by being a better man. If love is a matter of taking action, then that is love.