All the times they could have turned back.
I’m still blown away by how perfectly everything fell into place for these two.
There are so many things that could have and should have led them away from each other. Hiccup could have missed, Hiccup was supposed to kill him, Hiccup could have run away from the scene and left that dragon tied up in the woods - or even have run to his father and brought Berk to the scene of the crime to let them to the dirty work for him and still walk away with the life he always wanted as the captor of a Night Fury.
When he was freed, Toothless could have killed the human who trapped him, maimed him, and held a dagger over his neck. But he lets Hiccup go as well.
Now here’s where any sane human should have turned around and never set foot in that forest again.
But Hiccup is curious. Hiccup is passionate. Hiccup isn’t happy just walking away and never knowing what happened, so he goes back. And because he knows it’s all his fault, he works to make things right. He offers the dragon food, and drops his weapon at the dragon’s aggression. He stays in that cove for hours when the dragon shows absolutely no interest in interacting with him further. He could have backed down or left before the drawing-dance so easily. But he doesn’t.
And because they are literally reflections of each other, Toothless decides to investigate the scrawny kid who trapped, injured, then freed him, because he’s curious too.
The strength of the bond that tethers them was formed from such fragile threads.