You're used to it by now.
The first time she came over after fighting with Luke, you took it seriously, like this was it, this was the end of their relationship. You said all the right things and when she kissed you, you thought it was a big deal.
The morning after she was gone, and when you called her to see how she was, she told you she'd sorted things out with Luke, and that everything was okay. She never mentioned the kiss, so you didn't either.
The second time, the third time, you were still willing to believe that a fight meant it was all over, and that the kisses wouldn't be so easily forgotten when morning came.
Now you don't get your hopes up. When she comes over and cries on your shoulder and sleeps in your bed, her head on your shoulder, you know it's just temporary. You know that when she kisses you, it's not the start of something. You think maybe she means it at those moments, when it happens, when she leans in and presses her mouth against yours, that she believes in what she's doing, and that's enough for you, sometimes.
You know she and Luke will sort out whatever it is they've been arguing over, and that in twenty-four hours she'll have fooled herself into thinking that she really does have the perfect boyfriend and that nothing really happened.
If it were anyone else, you wouldn't let them get away with it. But she's your best friend, and her lips are the softest you've ever known, and you're willing to put up with it from her.