This was meant to be from the beginning. Even Sarah Michelle Gellar ships these two. This is the one man Buffy never really got over, and their love both tested and overjoyed her. Yes Angelus caused an intense amount of pain, but technically Angel and Angelus are two different people, which is important to note here. And even after the Angelus saga Buffy still loved him. Although they could never really be together, they're soul mates, and deep down they both know that.