I've read this a few times, and I'm wondering if it really is a happy ending. The last scene is obviously before the walk down the aisle; the bride still holding her bouquet and not wearing her veil is evidence of that. Additionally, the groom usually does not visit the bride just before the wedding actually takes place; the guy is also wearing what looks like a white suit, not the traditional black tuxedo. The girl's last line does not sound like a response that one would say to their fiancé, but instead a good friend.
What it looks like to me is that this guy never made his move on the girl, and instead remained beside her as a friend the entire time, up until the very end, where he is with her as a best friend on her big day. Maybe he thought she never saw him that as a lover, or just simply lacked the courage to reveal his love for her. If the author made a statement to the contrary, I'd like to see it.