Jack Frost, master of the hoarfrost and guardian of fun, had been alone a very long time. Despite finding new friends, becoming believed in, and defeating the boogieman; he would never forget what it was like to be alone. North, Tooth, Sandy, and even Bunny all took time to be together; to have fun with Jack, and Jack now had kids who knew when they were playing with him. This was more 'happily ever after' than he could of possibly dreamed of… and yet there was still that lingering doubt. Is this all?
Kids had many forms of entertainment, and Jack investigated each new idea with glee. Movies were especially good at instigating fun in even the most introverted child, and he had probably watched them all. Peter Pan was a favorite of his. A boy who never wanted to grow up, who only wanted to have fun, a boy who was just like Jack. When he first watched it he was astonished at the similarities, but shattered by the differences. Peter Pan had the lost boys - his friends, and children were more than willing to believe in him. Now Jack had those things too, but found out that he still wasn't quite like the children's tale. He had grown up.
Jack looked the same as always of course. Young, reckless, naive, and carefree, but learning to interact with people had given him a sense of responsibility. That was why he began a guardian, to take care of those who he shared his fun with. While the others may have noticed a new maturity in Jack, the change hadn't affected his crazy antics, they didn't worry about him. This feeling had given him the strength he had needed to protect the children and the guardians thought it might convince Jack to be more careful with his games. What no one realized was that fun loving didn't have to mean childish.
Three hundred years of loneliness and Jack still cared about people. Despite his invisibleness he interacted and understood people more than all the other guardians combined. He watched and learned and listened. Pitch Black had come first, and then the guardians. Though they had pushed him away, fear had remained and he bided his time to create an army of nightmares. This time they had let the Nightmare King be trapped alone with his creations. Pitch Black trapped in an endless nightmare of fear never to return. Most would say the ironic end suited him, but Jack knew better.
Jack understood fear. He had lived with it for a long time and even though fun could chase it away, it was a feeling that was never truly driven out. Some fun could be had in fear, scaring others, braving horrors, but fear had a more important focus. Fear kept you safe. It did not protect you, but made you protect yourself and others. Fear told you what was dangerous, what would hurt. Fear could be horrible, but even at its worst it told you what was wrong, what evil was. So how was the world going to get on without Pitch? Would children play with fire, hurt each other, or hurt themselves? Obviously fear remained if Jack could worry over these things, but maybe fear had a place in the world. Maybe Pitch and the Guardians counterbalanced each other. Or perhaps Jack just felt guilty.
After all, Pitch was a person and was alone. He didn't have fun to comfort him, he just had his fear. So either he stayed alone and miserable, or he'd come back worse than before. In the frozen wasteland Pitch said he understood loneliness and the fear to stay that way. Maybe there was more truth than manipulation in his words. All these thoughts and more he used to convince himself to fly to the boogieman's layer, but truly it was the idea that fear could be fun that let Jack walk into that dark place.