this is such a great question and i’ve thought about it nonstop…hmm lots of things, and im sure some of these are universally recognized as romantic but they came to mind nonetheless:

knowing what spoon someone likes to use for soup. kicking them when they steal all the blankets at night. arguing over who has to die first and then settling on going at the same time. as i once said about bts vmin and stand by generally: “i would choose you in the trolley problem no matter how ethically dubious”. laughing at jokes that aren’t funny in a way that’s clear you both know it’s a pity laugh but neither of you minds. when buffy and faith stab each other on buffy the vampire slayer. going back to a person after breaking up because you both realize you’re the only ones who can stand one another. long uninterrupted monologues even when they’re boring. “you’re so annoying 😍”.