In the episode "I Will Remember You", Buffy lost her memories of her time with human!Angel. But what if she hadn't lost them at all, or had regained them somehow? 

While part of her understands why Angel decided to change back, she realizes that his reasons were selfish; he wanted to change back because he felt the need to protect her, even though she doesn't need it. He tried to tell her that it was for the greater good, but Xander doesn't have powers and he manages fine, as does Giles. She also doesn't like how Angel tried to make her forget their time together; she realizes that he's trying to control her, and she let him make her feel weak and dependant on him. 

It makes her question her dating life, and how normal guys always seem to run away from her. Angel told her to find herself a normal guy, but when he was one, it wasn't good enough for him; he had to play hero for her. Because of this she turns Riley down and falls into a depression. 

Spike notices, and since she's impossible to annoy in depression mode, he asks what's wrong. She explains, and Spike tells her that she doesn't need a man in her life; she's a brilliant young woman who can stand on her own, and if she ever finds love, it should be for the right reasons and not because she feels that she needs a boyfriend. 

Not only does this cheer her up, but it makes Buffy see that Spike admires her strength. A later incident with the Initiative shows her that he doesn't mind her saving him. Feelings develop. 

Bonus points for Buffy smacking down Angel when he shows up in "The Yoko Factor" later on.