On the willows there, we hung up our lyres

Lan Wangji wants to say that he hopes Percival can be happy regardless of whether their god wills it or not. But he has already seen how directly that god meddled with Galahad. Their world is not their own. "I do, too," he says instead, and he is powerfully glad that he sits here with a young man who will never be the king of anything more than his own fate.