Funny thing is, he'd been doing that since before he even found Toothless as can be seen with the way he went, "Except for you," to that monstrous nightmare.

He rants and vents and rambles to them, he bounces ideas with them, has entire back and forth conversations with them sometimes, gossips with them too tbh, sasses at them, his entire mannerism of communicating with dragons implies that he is talking to a regular person.

Like, if you replace the dragons with humans in random scenes of the series where others interact with them, it would seem uncanny to watch because while not always, there's still sometimes that cadence to the voices and behavioral patterns that indicate this person is speaking to an animal. The rest of the gang gradually grows out of it but sometimes it flares up, with Snotlout and Fishlegs being the least likely to have that sorta tone (heck Snotlout even talks like the other dragon is his rival or something).

But no one has that same level of casual communication with these reptiles that Hiccup does and he only develops that soothing voice when a dragon is panicking or upset. If every scene of Hiccup's interaction with dragons throughout the series was made so all dragons were humans, it would look completely natural and nothing would be able to suggest that something had changed.

(A few examples here including showing exasperation at the wild monstrous nightmare, speaking to Torch throughout the episode, him and Toothless ALL THE TIME, "Meatlug, where are you taking me?", he was speaking to them all in gotnf when he found them with the hatchlings, "Not so fireproof on the inside, are you? Here you go.", ranting to that thunderdrum in Imperfect Harmony, speaking to it throughout and even at the end when the baby was reunited, "Yup, probably deserved that," when Smidvarg shook his eardrums, "Remember, not a competition. *To Sharpshot* This is really gonna go bad in a hurry.", "Wait a second, I know you," and "C'mon, guys, you're not gonna start this again. Right now, we've got a fire bearing down on us.", Nodding to Mama Whispering Death and then saying "Uhhh, you're welcome?" to Screaming Death, trying to talk to the Skrill in dob, talking to it in rtte, scolding Fanghook too when scolding Gustav, "No. I'm sorry but... Little guy, you need to go with your own.", boasting to the shadow wings by quoting Dagur.)

Been drawing dragons lately

Projecting my period cramps onto Snotlout or however the meme goes (How convenient he has a 5000 pound heating pad at the ready)