this person right here
their hunter x hunter tag for your viewing pleasure
The story of their meeting / early relationship is so sweet and intense (killua being gon's first real friend and gon rescuing killua from his family) that every interaction of theirs after that already has this love and depth to it
They have an interesting dynamic and they're both fairly complex characters who are situated in a fleshed out world and story, which makes it a more interesting ship than ships which are just "this boy and this boy are cute". It's also closer to canon than most m/m ships ever get to be (outside of straight-up BL/fujo series) which gives it a lot of appeal.
Also, it's probably the best shota ship.
I love that Killua is very earnest and open about his love for Gon from the start, even though he superficially has some tsundere traits. It's refreshing.