If you love someone, let them go. If they come back, they're yours. If they don't, they were never yours to begin with.
Erebor is being rebuilt and just in time for Bilbo and Thorin's wedding, which would be perfect, except for one problem. They're awkward together, they don't speak anymore, and they rarely see each other. It's nothing like when he first asked for Bilbo's hand, before they went to Erebor and...well. That happened. It's as if he no longer loves Bilbo anymore, and is marrying him out of duty or obligation.
And when Bilbo realizes that everyone else has figured that out except for him, he knows what he has to do.
One-shot.