Something I rarely see talked about is the fact that, at least twice in the first movie alone, Toothless helps Hiccup set boundaries or get the respect he feels Hiccup deserves.
He didn't start flying peacefully to show Astrid how nice it can be until after she apologized for being mean and threatening to tell the village. And when she did, Toothless almost immediately relaxed and arguably showed off. Like he was saying "look at this. Look at us. Look at what you'd be breaking apart and missing out on."
And, even if he didn't know the full story or reason, Toothless knew Hiccup and Stoick had a fight and Hiccup was upset. So he didn't show Stoick that Hiccup was alive until after he apologized for hurting Hiccup and starting that massive fight in the first place. In that scene, if you look at Toothless's eyes, you can tell he's waiting for something. And the second Stoick apologized, Toothless softened and opened his wings.
Even if Toothless didn't fully understand the entire situation or all the complex human stuff associated with it, he still knew that Hiccup was upset and deserved an apology. Even if neither time Hiccup asked for one and was ready to just sweep it under the rug and forget, Toothless still went the extra little mile to make it clear he wouldn't allow Hiccup to be pushed around or hurt or disrespected or anything without consequence.
Just. I love that. I love how that alone shows how damn smart the dragons, or at the very least Night Furies, can be. Toothless is Hiccup's number one advocate for anything.
