I guess it's like when you try to achieve something so bad and when you achieve it it's just meh and you realize that you worked so hard and sacrificed so many things (in his case) just for that one thing that's not even as good as you thought it would be.
He changed because he regretted sacrificing everything just for that one title that he didn't even get the way he wanted to get it.
Made me wonder while reading the your comments and something interesting was point out.
How when Endeavor got the #1 spot and realize the sacrifices and cruelty he inflicted to his family for that spot, for that one thing that's not even as good as he thought it would be. Maybe that was foreshadowing of this moment ?
Maybe Endeavor need to die by Dabi's hands and the tragic irony, would be for Dabi having the similar realization... doing ALL this to achieve a goal only to realize there is no fulfillment ?
You know thinking back I don't understand why Endeavor changed. Like he got the #1 spot then through a tantrum in his gym because getting it by default was not how he wanted to get it, then suddenly he just sort or realizes he should be better. Why?