Honestly, I think from the moment that Percy found out that not every demigod has a loving mortal parent they can depend on, when they already can't depend on the gods, and that Annabeth specifically, hasn't had anyone take care of her like that since she was 7, he decided he would be the one to.
They take care of each other up to that point, but I think that's when he starts noticing how much she takes care of them. She already knew if she didn't no one else will. She protects them against humans, she stands with them against monsters, she always brings up the rear of the group when they're running, and she is always the first one to make a move when they're fighting. She does it because no one is going to protect her if she doesn't do it herself. No one is going to protect Grover and Percy if she isn't.
In the arch, he pretty much tells her he would fight the gods for her, and then he goes on to do just that.
Athena, the one person who was supposed to protect her willingly pushes monsters her way, she lets Echidna and the chimera into a place that was supposed to be safe. Her decision is that Annabeth should be punished, and say what you want, but I think she knew that Annabeth would think through every scenario and know that someone had to stay and hold them back. And I think she knew that Annabeth would be the one to stay. Because that's the way she raised her, knowing that no one cared whether she lived or died. And I think that's the cruellest part.
Which is why Percy's sacrifice means so much more. He not only made the decision to trade his life for hers and fight the monster so she and Grover could escape, but he also made the conscious choice to push back against Athena's wishes. He fought a goddess for her, Athena said "Annabeth will die for her impertinence" and Percy said "Not today, not ever"
