Cause maybe she doesn't want to admit that he's not perfect, because he's not but she thinks that maybes he's perfect for and that's why they have to end.
So when she goes to his house puts on a fake smile and asks Kirsten where he is she says "he's upstairs'"
And when she saw him lying on his bed it almost made her want to re think this whole thing. Almost.
And when he kisses her she knows she has to tell him cause she cant say goodbye without telling him.
"I love you" she says and she watches his whole face change and when he's about to say something back she kisses him again.
And she almost certain that he's going to say it back but there was a chance that he didn't , and whatever he said could make or brake her and she didn't like anyone having that kind of control over her
And when she stops kissing him and looks at him taking all his features in, she needed to remember every little detail of him because she couldn't bare the thought of making love with someone else and not picturing his face. So she gets her right index finger closes her eyes and goes over his whole face
His eyes, his mouth, his skin. Everything about him was perfect, even if he wasn't.
And when she walks out of his room and says goodbye she's fairly certain that he knows it forever, and that's why she cries when she hits the bottom step of his house , because he doesn't stop her, but then there a chance that he didn't know.