“Astrid didn't feel any attraction to Hiccup until he became useful to the tribe” (c) an Eretstrid fan
That's what I love about most fans of non–canon couples httyd - when you ask why they like their couple, the first thing they answer is: “actually, I've never liked Hiccstrid because...”.
A war is raging on Berk – a lot of fire-breathing, flying reptiles attack the village, destroying crops, stealing cattle and leaving the Vikings without a roof over their heads.
What distinguishes Astrid from her peers is that she knows how to prioritize. Now, when her tribe is on the verge of starvation and utter chaos, Astrid chooses the welfare of the people, not her own. She doesn't need languid looks, she doesn't need butterflies in her stomach and serenades under the window, she doesn't need any of this. All Astrid Hofferson wants at the moment is the well–being of her tribe. So, it's not surprising that Astrid didn't consider Hiccup as an option. There is nothing surprising in the fact that Astrid did not consider anyone as an option.
For example, let's put Eret in this setting. It may seem that Astrid might well be interested in him – he is a tall, attractive man, he has all the qualities necessary for a Viking, and he also acts against dragons. But there is one caveat: Eret works for Drago – for a madman who almost killed the Chief of the Hooligans. Eret does not care about anyone's well-being, he catches dragons solely for the sake of his own prosperity, he grovels before the enemy of Berk. This is enough to make Eret unattractive in the eyes of a girl focused on protecting her tribe. Don't believe those who say that context doesn't matter, because context matters.
Okay, that's sorted out. Let's fast forward a bit to when the Vikings began to live in harmony with the dragons. Astrid can relax – now that the village is not in danger, she can think about herself. Astrid has always been and remains a warrior, but now she has much more time and opportunities for self-realization. Astrid tries her hand at dragon training, trains with Gromgilda and helps ensure comfortable coexistence between Vikings and dragons.
And that's when Astrid really pays attention to Hiccup. Over time, she begins to be interested in him not as a person who stands for the future of her tribe, not as a good companion, but as a person. Their priorities coincide – they create a new world based on harmony between Vikings and dragons. Teamwork gradually turns into a strong friendship, and at some point Astrid pays attention to what is happening between them. Something deeper than camaraderie, friendship, or superficial infatuation. She points out that her relationship with Hiccup has never been completely platonic, that there is a mutual attraction between them.
Also, I want you to understand that I have nothing against those who consider Eret and Astrid a great couple. I just want to say that there is nothing wrong with the fact that Hiccup and Astrid's attraction to each other did not arise at the snap of their fingers. Each couple develops individually, and most future lovers do not initially assume that they will be in love with each other.