Daisy isn't falling over Thomas anymore, and William is relieved; she defends him against the other footman, and he allows himself to hope; she kisses him to cheer him up, and he loses his mind.
He lets it stay lost, lets himself ask her if she will be his girl, lets himself propose to her before he leaves for army duties. But he has only lost it, and he knows what it is like to have it.
He knows that girls who are properly in love don't find excuses not to be alone with you, don't shrink from contact, don't look like hunted rabbits. He knows that at the end of this war he will have to make a choice, have to let her go or pretend not to know she wants to, have to be selfish or brave.
The war doesn't end. Not for him. In some respects, that is the easiest way.
He lies in a more beautiful room than he has ever dreamed of dying in, and because he won't be able to live them out, he lets himself imagine the most beautiful days, imagine her loving him for good and all, imagine a long and happy future. One night, awake and not trying to sleep because there aren't many nights left, he remembers the pension for war widows and hits on a plan that will see them married in the best way possible for her—briefly, but profitably.
When she finally gets past avoiding him and answers in the affirmative, she is trembling all over, and the tears that slip out despite her attempts to hold them back are not happy ones. She looks terrified, and he knows she feels terrible about lying to him, but he can't bring himself to tell her that her feelings are not a secret, that they are both pretenders. He can't bring himself to hear it spoken, harsh and irrevocable. Instead, he asks her to take his name, and she accepts with a quaver in her voice.
He chooses selfish, then, after all, at the end.