He's like a vaccine.
She needs to infect herself with the disease so that she can become immune to it. This is why she sits next to him. This is why she talks to him. This is why she remains his friend.
He tells her he likes this girl.
She tells him he should go for it.
He tells her he has a girlfriend.
She tells him congratulations. She knows they'll be good for each other.
He tells her he loves her.
She tells him she loves him.
She wonders if she should tell him that all she wants is for him to love her as much as she loves him. But then she stops, her hand resting over her heart, and remembers: she would rather hate herself if it meant never hating him.