Neville wanted more than anything - except to escape the whole situation altogether - than to explain properly. And he didn't want this, this panicked last-minute act of confusion, to be the last thing she remembered about him if he didn't come back.
He took a step forward and kissed her, putting all his pent-up affection for her into the kiss. He knew immediately that he had made a mistake because once he had started he didn't want to stop. But he had to. He broke away, hardly able to meet her eyes. "I have to go," he told her in an oddly broken voice, and fled the room.