(wrote this in a separate doc, in case you're curious how seriously i took this question. brought to you like neapolitan ice cream: in 3 parts)
BASELINE: 2pickship, same-age friends, glorious 1 subservient 2

COMPLEMENTS: born on opposing equinoxes (hhj spring, ksm fall) and thus both sister signs + cusps (hhj pisces/aries, ksm virgo/libra - water/earth or fire/air), dancer/singer, introvert/extrovert, red-coded/blue-coded, heart/head, bright-eyed main character/stoic love interest

THAT'S MY GUY:
(cancam magazine nov 2021 tr. seungdduk, tweets from thinkabtsj)
🥟: there are things that burn up and disappear, but seungmin doesn't disappear and keeps burning gently
🐶: there are many people who can dance well but hyunjin's uniqueness and ability to express is beautiful

(cancam december 2021 tr. inniefinity)
🐶, about 🥟: "we know each other's troubles and strengths well, a friend i can rely on."
🥟, about 🐶: "a rising comedian. my most interesting and precious friend."

(seungjin 2kr, 2022)
🐈, about 🐶🥟: "it's like they stand back to back against their enemies and fight together. that's what their trust for each other feels like. they stand back to back and never look behind them, they just go forward."

i think the best way to explain why they work so well for me is that they stand on equal footing. regardless of how the public perceives them as people/idols/teammates, they have the utmost respect and adoration for each other that no amount of akgaes and antis can tear through. where hyunsung are guys who are so similar they clash because they're confronting themselves in the mirror, seungjin are opposites who meet each other where they are and grow together, having changed because they're in each other's lives.
they tease each other and nag and have silly terms of endearment (who fucking else could make flour [boy] and rice boy meaningful nicknames like...), and they also drop the rawest lines about the depth of their affection for each other. "what do you even know" hits the way it does because it illuminates everything beneath the surface of the iceberg. stealing s's joke here but they're literally like hyunjin: "if seungmin has one million fans, i am one of them. if seungmin has ten fans i am one of them. if seungmin has 0 fans, i am dead" and vice versa.
ending on a personal note, as guy who is queer aroace, i've struggled all my life to wrap my head around romance and what a partnership, not to mention an ideal partnership, would look like in my eyes. my other favorite ships satisfy different curiosities, but seungjin are probably the closest to that ideal—friends who understand each other so well but could spend the rest of their lives getting to know each other. soulmatery prevails!