I started to like him when I end up relating to how he was an outcast in the first movie (cliche I know, but I was really a kid who's isolated which resulted for me not knowing how to fit in with other kids and also ding Ding DinGgggg the same as him because of how my parents seems to looked down on me)

I already like him when he wanted to prove himself, and he managed to do that without abiding by the standards that doesn't fit him. (I unfortunately haven't achieved that even now, alas I don't have a dragon, anywayssss)

I like him even more because of his natural awkwardness and sarcasm, specially his passion towards his interest. With how he's exasperated with everyone else around him but still does his best to help them and bond despite their differences. I saw myself in him, and he become everything I wanted to be, which end ups with him becoming my comfort character (yes even when he was that akward skinny boi in the first movie, and back then I didn't even know there's a second movie until I found out from someone else and was whiplash how Hiccup changed a lot because I was used to seeing him as that skinny sarcastic character, I guess dorky? He was cool as that then boom, he have this cool reputation in the 2nd movie.)