u'know, bulgaria used to be ruled by the byzantine empire for over 150 years (1018-1185). maybe during that time, he interacted with greece (who was still a little kid) and greece began thinking of him as a cool big bro (even though they weren't actually brothers)
bulgaria would probably teach him all swear words and offensive gestures (like middle finger etc, it's probably canon that greece mooned the crusaders during the siege of constantinople in 1204, guess where he learned that from, this was probably the only time when his mom didn't pull his ear afterwards)
this was bulgaria's petty way of getting back at the byzantine empire cuz he knew she'd get pissed off upon hearing her son use vulgar words (but then she'd ask her son "where did u learn that from?" and he'd be like "bulgaria" and she'd get FURIOUS, but u'know how bulgaria's a masochist and how he had a crush on her, so maybe all of this was a ploy to get her to hurt him lol, maybe she's the reason he's a masochist in the first place cuz he had a crush on her but they also fought wars and stuff)
greece in return would tell bulgaria any curse words he knew
and so the 2 of them would curse like there's no tomorrow
like imagine
the byzantine empire has locked the door of the room.
greece and bulgaria are cowering in the corner.
'NEITHER OF YOU TWO WILL COME OUT OF THIS ROOM UNTIL YOU TELL ME WHICH ONE OF YOU TWO HAS BEEN TEACHING THE OTHER THESE TERRIBLE SWEARS'
'I taught greece the swears HIT ME HIT ME'