#1. He's genuinely invested in the AJ Styles feud
Everybody loves working with AJ Styles. His in-ring talent is on such a high level that WWE, within his first year in the company, they put the world title on him and dubbed him as the best in-ring performer of this generation - a huge comparison to Shawn Michaels.
Styles being so good even prompted WWE to try to contact Michaels to have a return match with him, but the Heartbreak Kid refused to do so, wanting to honor his retirement (though he broke that in 2018).
The personal aspect of the rivalry with AJ Styles is what makes it so different and The Undertaker likely knows that this is his most interesting feud and match in many years. He could simply be motivated, which is what prompted him to make a major change. Even if he's beating AJ Styles at WrestleMania 36 (which we fully expect him to), he could still put him over without outright losing to him. AJ Styles deserves that much.
Also read: 5 reasons why AJ Styles 'exposed' The Undertaker on RAW last week