The answer to this question, I think, can be analyzed from the following aspects:
First, Luffy was trained inhumanly by his grandfather Karp since he was a child, thrown into the primeval forest, thrown into the canyon, released into the sky, as a child Luffy has already trained super vitality, and has also trained a strong ability to resist blows.
The second aspect, Luffy's demon fruit ability, Luffy said before, his grandfather hit him the most painful, because Cap's iron fist is not covered, the average person really can't bear it, and Luffy's rubber fruit, with elasticity, for some attacks, in fact, has the ability to cancel, although Kata Kuri used domineering, but there will still be some effects!
In the third aspect, Luffy set off from the windmill village, all the way to the world, he has met a lot of masters, long-term battle, for the actual combat experience, it is quite rich, know how to avoid the key points of nature, many fierce battles also trained Luffy super strong physique.
Fourth, Luffy's physique, resilience is extremely strong, as long as you eat a full meal can be full of blood resurrection, in the battle with Kata Kuli, Luffy also ate a lot of glutinous rice, which for Luffy, is undoubtedly in the congestion plug-in.
Fifth, the protagonist aura protection, as the protagonist, is a privilege, that is, the opponents encountered will eventually lose to him, although the early old beaten, but can not die, this should also be Luffy has been attacked many times and is fine one of the reasons!
As for why Kata Kuri was hit by a punch, perhaps Luffy's personality charm was appreciated by him, so he deliberately released water, of course, it may also be too much water for Luffy, let go, stabbed himself too badly, and may have met so many opponents that made him so laborious, so that his physical fitness was not very good, so it was easy to lose.
In short, Luffy's strength and persistence are commendable, and Katakuri's benevolence is also worthy of applause.