It is so tragic, how proud she is of being asked to be a girlfriend.
I actually do get this. In her mind, he did not choose her as his wife. He didn't even know her. But he did choose her to be his girlfriend even though he knew her flaws. So she thinks he genuinely chose her.
However, what Bec doesn't see which the rest of us do, is that he doesn't mean it, he's only doing it to make her happy. That's why it means so much to her and so little to him.