It was not Hagler's mentality to go to "war". He preferred being a counter-puncher, even though his physical attributes allowed him to be a great aggressor also.
He had to basically convince himself through an intense training regimen and a promotion job that he had to brawl with Hearns in order to make an impression on the people who had criticized him for performances such as the Duran fight.
Hagler obviously never enjoyed being hit and tried to avoid taking unnecessary punishment, despite his great durability. It's no easy feat to convince yourself to walk through a Hearns right hand but in this case it was going to be easier for Hagler than trying to box with him and he followed through in a big way.
Hagler was also thrown off by Duran's tactics as he expected the smaller man to come right at him which would've allowed Hagler to play a more comfortable role without having to go after Duran himself. He was just never able to truly adjust until Duran tired in the late rounds.
By the time he fought Leonard, Hagler had slowed down quite a bit and had the foolish notion that he would outbox Leonard early, which didn't prove to be a smart strategy at all. Leonard avoided Hagler's power for the most part and had the toughness to hold out for 12 rounds despite his legs starting to go at the end. If Hagler had gone at him from the beginning, things may have been different, but that's just one of the many big 'ifs' of the fight.