Solitude
There are days when she disappears, and he hunts high and low at first, with increasing desperation, terrified that he's lost her again. The first time, it's Astrid that finds her, on Dragon Island, staring out to sea. She smiles and returns to Berk with the younger girl. The second time it happens, Gobber finds her, and she follows him back to Berk too. He scolds her, and she apologises to Hiccup when he greets her, face pale. ("I'm never leaving you again while I still breathe, I promise you.") The third time it happens, he finds her himself. He wants to ask why she keeps vanishing with no warning, to beg her not to leave again, but something stops him. Instead he sits by her, leaning back against Toothless. Gradually he relaxes, stresses and strains fading away, and as the sun sets, it dawns on him. Berk is noisy, full of people and demands. Out here there are only dragons and the open sky. Freedom. They share a smile, and she follows him home.
The next time she disappears, he doesn't search, just smiles and shrugs. "She just needs a little solitude. She'll be back."
And that's all that matters.