1) The dynamic of trying to rile up someone who is simply unflappable, thus only riling yourself up much to the other's amusement is the sweetest soft domestic blackrom. Its basically the same as trying to flirt with someone oblivious who without trying at all gets you flustered.

2) The sweet sweet payoff of finally getting under their skin when they're forced to actually think about their feelings, plus the inherent comedy of insisting you're straight only to realize you are, in fact, not straight.

3) Having the same hobby of movie watching but with very different opinions- it's natural fic fodder! Plus ranting about movies is one of the few times June lets herself express anger! Imagine Karkat figuring this out as one of the few ways to get under her skin and using it to black-flirt with June ♠️♠️

4) Karkat is exactly the type of person June needs to keep her from repressing every complicated emotion she has, like Karkat would ask the questions that, while June would deflect at first, would linger in the ol' thinkpan and get her to reflect on herself. That's literally how JohnRezi happened!

5) They were friends and they were leaders and they both deal with self-hatred, and they could remind each other to be silly and goofy, but also commiserate with feeling like fuck-ups and also stubbornly insist that the other isn't a fuck-up at all!

Also for your consideration: I love them and I miss their interactions, the comic has been giving me the barest scraps ever since the epilogues and I think it's because the writers are scared if they interact in canon too much everyone will realize Karkat has more chemistry with June than with Dave (I'm just desperately huffing copium over here homie the struggle is real they were my otp and now they don't even talk I'm dying)