Tsuna was good for him. Hibari knew that. He'd been much quicker to anger before Drifting with Tsunayoshi. Now the only thing that really, truly pissed him off like he had been before Tsuna was someone going after Tsuna in the first place.
It was a novel experience to have a longer fuse. Tsuna calmed him while he brought Tsuna's temper closer to the surface (and no matter what Tsuna grumbled at him, Hibari stood by his decision that this was a good thing).
Fighting by Tsuna's side was….something else.
Hibari can't ever explain how he had felt when it dawned on him that Tsuna was a Carnivore in sheep's clothes.
He had considered this man weak at one point. Dubbed him an herbivore and been done with it.
Sharing his mind had shown Hibari he was wrong, but fighting with Tsuna, sharing thoughts and feelings as they lunged to meet the Kaiju Ironhide… feeling the way Tsuna's instincts had sat up and taken notice of the world as if they were Hibari's, knowing the how and why Tsuna thought as he did…that had ripped every shred of doubt that Tsunayoshi was a Carnivore right out of his mind and left him a little ashamed he had spent so long dismissing and doubting Tsuna. (Hibari did not like feeling ashamed),
(Tsuna had snorted at him in their next Drift when he picked up on the feelings, pointing out that everyone had done it from the young children right up to the adults in his life, and Hibari was making up for it now.)
(But Hibari wasn't everyone. He should have noticed.)