Usually, at this point, I'd think this Tsuna-guy would be the guy I'd despise. But this time, albeit I think 'watersport' is beyond my interest, it is how the "animal/canine culture" is. While Taika thinks to be friendly by butt-snuffing, Tsuna rejects the concept of being friendly (maybe because he's taught that way in the police), and instead showing dominance, as still-animal-minded-Taika would understand.
Taika: "Let's be friend!"
Tsuna: "No. I OWN YOU!"
Although, in human world, such dominance is usually shown through bullying. And if they're both human/oid with human morals, I usually will mark Tsuna as the bully and despise him.