In the context of the story, how should Valka react, after being isolated from meaningful human contact for two decades, to the sudden appearance of two personally significant characters and the revelation that Berk had capitulated to her ideals? This last point in particular seemed like an impossible feat to her, and sufficient grounds to justify a hermitic existence. For Valka to take such world-shattering news in her stride would seem superhuman.

So she's a bit winded, and Stoick's picked up on this. What should he do? He's no wordsmith. He draws on a powerful memory that he shares with Valka. A joyous memory: that of their courtship. A simple tune, some plain lyrics (for Stoick's no songsmith either), and you've got yourself a DIY psych session. This series of events seems plausible.

In terms of storytelling, we needed an upbeat sequence to transition between two downbeats: "the bad guys are coming" and "the bad guys have arrived". With that in mind, what we're left with seems fitting: it delivers a moment of levity, fleshes out Stoick and Valka's backstory, establishes "the romantic" as a facet of Stoick's character, and sets us up for the fall (when Stoick dies).