Tbh, with that Luffy vs Katakuri fight, I always thought that not only was Luffy using Katakuri to grow out his haki and get FS and stuff, but Katakuri was also using Luffy to also grow out his haki as well, since "haki blooms in tense situations".... I mean, dude already got FS, but he wants to become even more stronger than this.
and his sudden 180 to Luffy - with him fighting man-to-man - is coz he saw Luffy as a worthy opponent he could fight to grow even stronger, seeing how the guy dodged his attacks, and him not getting hit in a while even though it's coz he lost his cool for a sec.. etc.... it could be why Katakuri let Luffy go snakeman at this point.. he saw nothing in him before, but then he saw something, and used this opportunity to grow stronger.
I mean, if you look at it one way, Katakuri was defo fighting strong guys in the past -- I mean, how else did he get such strong CoO like that?? -- but later on, the guy was just fighting scrubs not worth anything; seeing Luffy as a worthy opponent, it was like.. his chance to become even more stronger once in a while.
I mean, idk.. he could just fight YC's to get stronger, but how often do YC's encounter each other??