Feel free to tell me your thoughts about this, though, because this is my first post. It may not be very good, and if it's not very good, please tell me what I can improve so I can get right on it! Anyway... on topic.
Finn the human and Jake the dog are the main characters in this story, and watching over the episodes you'll notice several things relating to previous human civilizations that have been destroyed by this war. This might come as a surprise to people who have not researched it, but the entire story of Adventure Time takes place after a nuclear war, in which it seems almost everything was destroyed.
The reason why Finn survived remains unclear. There is a post on the Adventure Time wiki telling about this: http://adventuretime.wikia.com/wiki/Mushroom_War. It gives further proof on these theories. Now that you know about this, in case you didn't, it's time for my personal theories.
The Great Mushroom War was believed to have been a nuclear war that had devastating effects, as in it took an entire chunk out of the Earth. As it would seem, Finn is the last human, possibly next to the Ice King. There is no proof that the Ice King is still human, though. The episode "Holly Jolly Secrets Part II" shows how the Ice King became what he is, and due to his supernatural powers he was able to survive the war. In my opinion, and possibly others, The Great Mushroom war is referring to the shape atomic and nuclear bombs make when they explode. There is no proof of this, though.
My theory begins here. There's two split theories here. It is possible the nuclear war had devastating effects on Finn, as in he lost everything he had ever had. His family, everyone he knew and everything he loved. When the nuclear war happened he became mutated and had lost track of reality. With his devastating loss, would it be possible he made an entirely new world and life so he wouldn't be alone anymore, so he could have everything that was taken away from him back?
It seems very possible to me, that he created everyone in the series to fill the places of those people he lost. There is a flaw in this theory, though. There's an episode that shows Finn as a baby, and everything is the same way it is in the present. My idea might be right though, but it's hard to fit it in to the story. The second theory though, is that it all did really happen due to the nuclear war, which isn't a theory, but still. Anyway, this post isn't perfectly laid out, and if you're wondering why it's on Creepy Pasta, it's because I found it creepy. :3
Whatever. I'm done now.