He had been alone for so long.
Well, that was not true. He had his puppets. His big crate of puppets. But puppets did not speak; they did not hug; they did not smile. At least not outside of his imagination, his broken mind.
At first, he had pounded desperately at the door and crying out: "You have forgotten about the other dwellers!".
No one ever answered. The door didn't budge. He was alone.
Realization slowly set in. They hadn't forgotten. They had locked him up in there on purpose.
He didn't even remember his name any longer. He didn't know how long he was locked up. All he remembered was entering the vault and then the door closing behind him.
Weeks went by, then months, then years. He was desperate from the beginning and discovering the crate of puppets only made it better for a while.
He created stories, gave them each a personality and stories. He tried to cope.
Nothing worked. Puppets began committing murder, destroying one another, and the puppet man didn't realize it was he who was killing them in reality.
He eventually made it out, but his mind never did. It was still locked inside Vault 77, always and forever, his sanity lost among a crate of puppets.