They once were one country...
Once, both countries were one nation. But then they started and lost a war against their neighbors all around. As part of the peace deal, they got split betwen the fragile alliance of nations that had won. But the alliance was not to last for political reasons. So the ex-allies did errect two totally different governements. Let's call them the north and the south.
The north was recreated by the Northern Giant in the shape of a one-party dictature. It was created as a border-marsh, the people were made wary of 'the enemy' right over on the other side of the border, people that were friends just years ago. Political indoctrination fortified the minds over the next 40 years while The Party did dictate economics and culture. Life in the north isn't the most happy, as they are pretty much a subsidiary of the the Northern Giant, who set up the state, and they look at the south with envy.
The Southern Giant reeducated the people to form a democracy like them. It sheltered the fledgling state and educated them how to not fall back. The new trade made its economy flourish. First they made them a junior-partner in a defensive alliance, then they gave them the means to become a proper partner and they did. Southerners however never stopped to demand that the Northerners should actually be part of their state, because that was what the provisional peace deal said before the alliance broke apart.
The Northern Giant however wasn'tas stable as it looked. Over the years it didn't age well. Its dictated economics ended up broke and destroyed itself within 35 years, the whole state was falling apart. And then a new head of governement came up that wanted to end the rivalry with the Southern Giant because he was fed up with building ever bigger and better weapons to get the potential of wiping the planet another 5 times. It was time to CHANGE. And change cam e radical: within a few years the puppet regimes the Northern Giant had errected around itself as a border started to crumble and collapse, and among them was North.
Northe, super proud about its (fake) achievements just had had it's 40th birthday with all the pomp it could muster... and then the shockwaves hit. Change was in the air, the party knew it could not fund their state anymore and they would go under in a fireball. And then they looked back at their hated neighbors and... simply said "ok, you want reunification? Let's make a deal. One condition though: We don't want our state to become 41, so make it quick!"
Within less than a year, the two states became one... The economics of North had to be overhauled completely, but... in the end, the two became a somewhat happy whole again.
Ok, who still hasn't gotten the huge clues, this has actually happened in our world. Because this is pretty much the story of Post-World-War Germany. North is a stand in for the DDR, or eastern Germany, a puppet of the USSR. South is a stand in for the BRD, western Germany, while the Southern Giant is a stand in for the USA, France and the UK.