Ponies get drunk, right? Some of them wake up with a hangover or even next to somepony and then proceed to run for the hills. This may go double if you end up in Las Pegasus...weird things happen there.
Our dear human, Jason is one such being, but his drunk mistake has upped the ante by being with none other than Princess Celestia and by that action alone they are forced to marry soon by a law long forgotten from Celestia's teenage years. But that's not the only surprise in store for the pair of them as many conflicts will be coming over the months ahead.
Whether the two like it or not, they must be together until Jason's mortal life ends and there is no way of getting out of that but to live with it.