If you're the champ, and the top five in your division remain the top five over a given length of time, rematches are inevitable.
If the two guys in question had an epic battle, or there was some sort of controversy, rematches are inevitable.
But I don't like immediate "gimme" rematches just because one guy was champ. IMO, the rematch really should meet some sort of the aforementioned criteria.
You lose the championship, you go pound on the rest of the division, then if warranted/earned, you get another shot.