Ron was jealous and awkward and silent. His every emotion was so transparent to her, and she found she didn't care to ease his discomfort. Harry needed her far more than Ron did. Actually, she often doubted whether Ron needed her at all. Wanted her around, certainly. Held her in high esteem, obviously. Would be upset if she were to disappear, surely. But he didn't need her, and she didn't need him as much as she loved him. Without noticing she'd made her choice, and Harry would be her first priority without a doubt.