IT WASN'T BUFFY THAT GAVE IT TO HIM. Certainly, she made it possible for Angel to experience that one moment, but the pure happiness didn't come from the one particular girl as much as it came from Angel's finally realizing that
I don't see the point of your point, I kinda agree with you, but it couldn't have been with anyone but Buffy, so either way it's still because he was with Buffy.
And it can happen again, the effects of happiness are artificially stirred in him with drugs in Season 1, and he looses his soul till they wear off.
If he could be with Buffy, he would be as happy again, and the same thing could happen. If happiness could be created artificially reached, there's no reason he couldn't get there naturally again.
And actually it's done again in Season 4 of Angel, where he's tricked into dreaming he was happy and loses his soul.
I also agree with the others that said Willow, just redid the curse, which full reactivated it again. By the time she had the power to do something different Angel wasn't in her everyday life so it kinda seems like she wouldn't have thought about it. Similar to Amy, she only kinda realised she could help her randomly, when she could have helped her for awhile, and then she started to spiral pretty bad.
Then when she re-souled Angel in Season 4, she wouldn't have had to confidence to mess with the spell, it was probably a relief that it was the first spell she'd done, no real danger.
I'm not sure id the circumstances of the curse are a mistake though, the Gyspys cursed Angelus so he would feel bad, if he could be happy what was the point of the curse.
WA I wouldn't be to quick to assume there's no "Escape clause" with Spike's soul. Spikes soul was restored by a demon. Possibly if you look at the evidence a vengeance demon.
From Season 7 "Selfless":
D'HOFFRYN:
I figured I'd be hearing from you soon. The flaying of Warren Meers? Oh, truly inspired. That was water cooler vengeance. Lloyd has a sketch of it on his wall.
Since there are those sketches on the wall of the demon that restored Spikes Soul, sounds like Lloyd restored spikes soul.
Demons don't really mix, and with these demons discussing it, one could assume maybe Llyod is some kinda vengeance demon. Maybe if Lloyd dies, spikes soul goes to, or if something (like Anya's necklace) of Lloys is destroyed Spikes soul is gone.
Plus remember how Spike went about getting his soul, before he left he said something about "How he was gonna teach her a lesson" - obviously talking about Buffy, In my opinion he wanted to prove that she could love him, and that the only thing in the way was Spike not having a soul - That sounds live vengeance to me.
In fact, maybe thats how he found out about Lloyd, he could have asked Hallie, they'd met up again earlier that season, he calls her up wants some vengeance, she specialises in children, so she points him to some kinda vengeance demon for demons.