Taking from another thread why was Marvin Hagler so good in remtches?

Because he battered em that much the first time around.
Same goes for Marciano to a degree.

 

One slight (double) correction:

It’s Sugar Ray Seales, not Fields, and Marvin won a 10-round unanimous decision in their first fight and fought to a draw over 10 in the second. He got the KO in their third match.

(The second fight, Seales’ home territory of Seattle, has the most bizarre scoring I’ve ever seen in a fight — two judges had it 99-99, so one round each with eight even, and the third had Marvin ahead by a couple of points.)

 

Hagler was better than almost everyone that he fought. I'm not sure if he was ever the underdog in any one of his fights.

If he was on his game, he'd usually beat his opponent handily, and there would be no clamor for a rematch. If he was off his game, then we might get a surprisingly close fight, or even the occasional Hagler loss. Then there would be some demand for a rematch, to see if Hagler could redeem himself. Hagler was a consistent fighter who was rarely off form, and it was always likely that he'd revert to his usual self for the rematch, which meant a beatdown for his opponent.

There were other factors at play (his rematches against Hamsho and Obelmejias happened because he'd more or less cleaned out middleweight and they had to repeat challengers), but it was mainly just that the middleweights of his era were always going to have a hard time putting together two good performances in a row against a fighter of Hagler's caliber.

 

It's actually very simple. Hagler was a smart fighter and very adaptable. He was able to analyze the first match and improve his strategy for that particular opponent.