Yes, even though I slept through TTT at Entmoot, I have seen it. ;-)
I got the impression from the movie that it was a dwarf thing that he didn't want to ride. Not just that he wasn't a good rider, or that he didn't want to ride such a great horse.
I don't remember him saying outright in the movie that he didn't ~like~ to ride. Remember a scene? Maybe we'll all have to watch again ;~D