Natsuhime : K, listen, I'm about to tell you the truth about Bulgaria. He might kill me for sharing this, but it's worth it.
Bulgaria is really shy, because he doesn't really socialise much, because of a certain accident. First, his mother died way before he fully developed as a country and ancient Greece had to look after him.
Second, when he moved from Russia to the Balkan, he met a really cool guy there, Slavia. They became really close, like extremely close, but unfortunately he died when the Ottom Empire invaded him. Oh, I forgot to mention that Slavia was bullied by Greece and Bulgaria was the only person that helped him.
Third, when Bulgaria found out that Slavia had children(Serbia, Bosnia and Herzegovina, Albania, Montenegro, Slovakia and etc.), Bulgaria wanted to do protect them with his life, but was unable to, especially when some other countries decided to fuse some of his children together, forming Yugoslavia. That caused Slavia's children to suffer and Bulgaria blamed himself for their suffering.
Forth, when Bulgaria joined Russia again, there he tried to forget all his misery, but failed miserably. Russia tried to help him, but failed, thus making Bulgaria snap at every mentioning of Slavia. Then he met Romania. Even though he didn't look at all like Bulgaria's fried, Romania acted just like Slavia. That's when he opened up a little to Romania.
Fifith, even though that was enough misery for Bulgaria, he suffered even more. When the Balkan wanted to fight against the Ottoman Empire, Bulgaria became a massive battlefield. When the Balkan won, soon after that, they all turned against Bulgaria. They took most of his land and Bulgaria was left broken. He was betrayed by Romania, but he forgave him after that, but he never forgot how Greece (Bulgaria considered him a brother) took an advantage of his weakness and stole some of his lands.
All that misery, but the greatest of them all, Macedonia. You know why? Because she was the closest thing that Bulgaria had together with Slavia. She lived together with Bulgaria and Greece, but then she left them and declared herself a contry. Bulgaria was yet again stripped out of land and you know what he did?
He was one of the first countries to recognise her as a country.
Oh, and that time when Bulgaria hit Italy?
Slavia could be considered Italy's uncle and when Bulgaria found out that Italy didn't lift a finger when the Ottom Empire invaded Slavia, he was so furious, but he knew that revenge won't do good, so just let it go, thinking that Italy was just too weak. But, when Italy fought the Ottoman Empire and won, Bulgaria just thought of hitting him but refrained from doing so, only sometimes Bulgaria thinks of Italy's face as annoying (bcz it was somehow similar to Slavia's and Bulgaria didn't want to be reminded of his 'friend') and he hits him from time to time.
There's this one gift that Slavia gave to Bulgaria and that was his knowledge and alphabet. Bulgaria didn't want to keep the gift to himself, so he dived it with the Slavic countries (Russia and Poland are also slavic countries) , preserving Slavia's memory.
The memory of his only loved one.