I think there are a number of reasons why Angel wouldn't want to become human before the fulfillment of the Shanshu Prophecy. The primary reason he refuses to accept it in
"I Will Remember You" is because he realizes that his value, to Buffy and to the world, is greater as a vampire with a soul than it is as a human.
This isn't delusion or ego talking. He's objectively correct. It's possible that, with time, he might have learned to be a great force for the cause of good, but as things stood in that moment, he was weak and unable to provide the support for Buffy or the lost souls that he protects. His body had changed but his mind hadn't, so like someone who injures themself, his mind needed time to catch up and stop acting like he still had his vampiric abilities. He was slower, weaker, and less durable than he had been, and it nearly cost Buffy her life. That was what drove it home for him. Prior to that, he was willing to work through whatever he had to because it was a dream come true, because it meant he could be with her, be happy with her. After that, he found it intolerable to think about living that way.
@hoponlilmama mentioned Angel's refusal to keep the Gem of Amara in
"In the Dark,
" which is another example of him giving up the ability to "live in the daylight" like a normal person, and I think part of his reason here is the same. He explicitly said so, after all. He names the objection to be that he can't be there for the people who need him if he has the ring, not because he's mortal in that case, but because he would be able to (and thus would inevitably) live in their world (the daytime) and would end up neglecting the people who desperately need his help in the dark, when there is no other help. I do think he has a second reason, though it is not stated—that is, the fact that it isn't real. It would allow him to walk among men, to blend among them, to pretend to be one of them, but that would be all it was—pretending. He doesn't want to pretend at being human. He wants to
be human.
There's a secondary reason that Angel doesn't accept such loopholes as the Gem of Amara or the Mohra demon blood, in my opinion. That is that he doesn't believe he deserves it. He looks forward to the fulfillment of the Shanshu Prophecy as hope because, when it happens, he'll have earned it, and that will be decided not by him, but by an external force, the Powers That Be. But in the present, he has no illusions that he has, even though by anyone's standards, he has done more for the world than could ever be expected of him. But, as he tells Faith, he believes he may never make up for what he's (Angelus has) done, meaning he can never be forgiven, never deserve a reward for what he's (Angelus has) done.
Angel: The truth is, no matter how much you suffer, no matter how many good deeds you do to try to make up for the past, you may never balance out the cosmic scale.
I also dispute the premise that the reward was his primary reason for doing good. It certainly wasn't to begin with. No, I don't think he does good so he can "become a real boy." I think the idea that someday he might be human again isn't his reason, but rather his motivation, and there is a difference between those two things. I think it's what gives him the hope to keep going, the hope that someday all this pain, all this hardship, all this suffering will end, will be rewarded. His reason is that he wants to help people, wants to make up for Angelus (even though he has no responsibility for that, he
feels responsible), but his goalpost, the thing he can focus on to push him through is the reward.
Anyway, those are just my couple of cents, for what they're worth.
🙂