It’s that look of Toothless’ in the 4th gif that gets me every time. That pause after Stoick say “I’m so sorry.” It’s like Toothless understands, knows that Stoick is apologizing to not only Hiccup, but him as well for his resentment of dragons. It’s like Toothless can understand that the broken bond between his rider and Stoick has been mended.
EXACTLY. Toothless is super sensitive of people, especially when they say sorry to his Hiccup. He’s protective of Hiccup, but he will let down his guard when he sees remorse or compassion. In this case, he saw that Stoick was sorry, like you said, for his misjudgment of Hiccup and for dragonkind. Toothless is perceptive of that in in this moment chooses to reward Stoick with his son, but only after Stoick recognized his lesson and humbled himself enough to understand it.