She listens, watching his face for any tells that this might be a joke. It's not. He looks pained, and part of her realizes that he probably is. From the way he looks at her it's not hard to tell that he still has feelings for her, and part of her wants to reciprocate them.
Oliver isn't going to change. He isn't going to one day turn around and realize that he loves her and he can have a relationship with her and be The Arrow. Any hope of that is really just a pathetic pipe dream. She knows that, but still it's hard to move on from that hope. Once she's managed to move on from that hope, well, it was easy to see why she'd choose Ray. He was kind and smart. He was honest.