when a soul mates

Death saves the day

There's this small important thing called universal balance that weighs on the shoulders of one cosmic being called Death. Death isn't just someone though. Death is an existential state that every created being can go through, and some have to. Like humans and animals. The less advanced kind of species. That doesn't mean they are less significant than other beings though.

Oh no.

Death honestly had a lot of respect for them. More than for other species. Because they are born and they live and then they die. And they know it. They live knowing they are supposed to die and that they can die of absolutely anything, very easily. They aren't as powerful as angels, demons or other hell-abominations and they still live their life and keep going. They always do.
Secretly Death loves their little motivational quotes like "carpe diem" or "YOLO", it's stupid but it's pure.

So when Death died when Dean slayed him, he wasn't shocked, he was sad. Sad to leave his job and to give it to someone else who would've never fit into his shoes.
He was the Death. How was a reaper supposed to reach up to that?
Well, turns out: no one did.

When he was in the Empty there was absolutely nothing. He wasn't awake or asleep and he couldn't dream of any of his remorse, because he didn't have any. He was Death. He was a purpose and not just a being with regrets and feelings. He was just a thing that had the uttermost control above all.

With no death there wouldn't be souls in heaven, the empty or hell. They'd just stay where they are and the world would get crowded of every being to ever exist.
Death decided who had to leave and who could stay. Even though he never really abuses his power to do so. There's always a time for everyone to go and die, Death just knows when the time was.

Billy died and also did Betty, the last reaper left.

Billy did a really poor job in being the controller of the living, she abused her powers and couldn't appreciate the meaning of her power. Betty had like ten seconds so she doesn't really count.
That leaves the horseman. He actually did do a good job. He was the real one, the real Death and he was able to value his position.

Right after Jack absorbed Chucks powers, he was now the so-called God. The creator of being and the light in the universe. It was a terrible burden to put on a literal child. But that was just his destiny, was it? He was always born for something big. His Mom knew it and Castiel saw it.
Being God meant to have the responsibility of everyone and everything. Hard do admit, but it was just a question of time until Chuck had gone crazy. The difference to Chuck was that Chuck wanted to get involved in every matter, Jack doesn't.
Free will was the number one thing he had to protect at all costs, even though the cost was living a normal life. Going back to his family, bringing back Castiel, it would interfere their lives once again and since he was God, if he wanted to or not, he couldn't do that to them again. They needed to make they own choices, without God being a part of it.
So that's what Jack did. Be absent.

Until he realized the world was a doomed place without anything dying in it. Life was supposed to end at some point and not go on forever. Especially since not even the monsters were dying anymore. No one kept coming to hell or heaven.

So that's when Jack did intervention number one.
He brought the horseman back because he was supposed to be Death. The real one.

It was a very hard time for Dean and Sam. First they were lost Castiel, second the entire world population got erased but them and Jack, when he was still just him, third they fought God but lost Jack right after and then they had to deal with yet another fight, that seemed to be endless.

The world got crowded rapidly, you wouldn't even guess how many people are meant to die in one day alone and when they don't ... well people still got born so they just became more and more.
It even got worse, when demons from hell got the news that they couldn't die anymore up on Earth, so they escaped one by one and found themselves vessels who also couldn't die, even when Sam and Dean stabbed them -or worse- so that became a terrible mess.