sirius has engraved sirius/serious jokes into regulus' very being; since reggie was little, sirius has been making them and encouraging regulus to make them, but even though regulus no longer thinks they're funny, he still makes them becuz of instinct.
baby regulus telling a baby story:
uncle alphard: wow are you serious buddy? did that happen?
baby regulus, stopping in confusion before pointing at himself: i reggie.
regulus, meeting the marauders,
james: what, are you serious you wanna be in slimy slytherin?
regulus: im actually regulus. but also yes.
james, half laughing: ur giving me mixed signals on how cool u r
remus, sitting down at a table in the library, regulus happens to be occupying:
regulus: wait u cant sit there
remus: im not in ur space, ur not serious are you?
regulus: no im regulus. but no barty was practicing weird charms on that chair
no matter how serious (hehe) or important the situation, if someones says serious, regulus has to make it into a pun. he cant control it.
regulus, actively dying: i just need a drink
sirius: r u serious right now u dumbass
sirius, choking up on laughter and tears: not the time reggie