Note: The story follows after the movie, tweaking the revelation of Jack's past a little bit. And a little warning for you, there are spoilers ahead! For updates, ask me via Tumblr and/or WordPress, username is djmarinizela.

"Jack, are you sure you want this?" Tooth, the Queen of the Tooth Fairies and the Guardian of Memories, imposed a sudden concern for him, not that she was always worried, but she wanted to make sure Jack knew what he was doing, especially when the topic involved teeth, be it in a comical, romantic or dead serious situation.

"There's nothing else I want more than to remember who I was, Tooth."

That was Jack's final decision and Tooth knew nothing could change that for he was a stubborn boy who followed his own ways. With a sigh of defeat, Tooth brought him to her palace. She led him outside the garden where a huge mural of Tooth receiving the children's baby teeth gazed down upon them. She opened a secret passage and drawers of all shapes and sizes emerged from the wall. She ran her tiny fingers across them until she finally took out one, revealed a small container, and held it out for Jack to see.

Jack was astounded with the small box that Tooth had in her hands. It looked more like a cylinder, in fact, with an intricate design of gold patterns weaving across the surface. On the side was a drawing of a child with brown hair and blue eyes. It resembled Jack and it struck him.

"This… is me?"
"Was."
"Oh, right. And you're sure this holds my past?"
"I'm not the Guardian of Memories for nothing, Jack."

She looked at him with utmost seriousness but said nothing else.

Tooth took him inside a large glass cage and told him to close his eyes. Before he could ask what for, Jack fell into a state of drowsiness and soon after, he was asleep. He was having a dream, as commanded by Tooth using the box of Jack's baby teeth. At first, he thought he was back in the modern town of Burgess where he spent hours looking at the statue of the founder of the town but he realized the statues had come to life. He saw his family, his young mother who was in the clothes of a Puritan and his father, Thaddeus Burgess, who built the first log cabin in Colonial America. He turned his eyes towards his seven-year old sister, her hair in braids, grasping her mother's skirt and looking at Jack with fear.

"…Lily?" He called out to her, but she cowered away from him.

He wondered why, but as he sought to think of an answer, his dream shifted to the freezing pond on the night he discovered his powers. His sister was in the middle, the ice cracking beneath her, her wails getting louder and louder.

"Shh, don't move, Lily, I'll get you out there in no time." Jack turned around and saw a younger him, his hair still brown and his eyes like that of chocolate. He went looking for a stick long enough to pull her sister out of the breaking ice despite his shivering figure. When he found one, he went back to her and with all his might, catapulted her sister back into the snowy ground where she was safe. And that was when the ice cracked beneath him. Jack heard the last words from his mouth, "No!", and after that, nothing. He knew he was dead that time.

He was still sobbing like a little boy when Tooth woke him up, her tight embrace reassuring him that everything was going to be fine. He had just learned bits of his past: his mother had died of tuberculosis a few months before he died, leaving his father a complete drunkard despite being the founder of the village until his death of old age. Her sister lived and had a family which continued their generation, but nobody listened to her story how her brother had disappeared under the pond for they found no dead body where she said should have been.

"All my memories… they've been here for so long, and I never even knew." He wanted this and he had no one he could blame. He didn't know what to feel, for he was heartbroken and yet free from the longing of knowing his past.

Jack didn't need to hear from Tooth that indeed it was painful, all memories he has lost rushing back to like a great tidal wave, engulfing him and his entire being. He didn't want to feel weak, and moreover, he didn't want to fear, because it would be the last thing he would want, knowing that Pitch dwelled on people's fear for his own power.

"You're stronger than you are, Jack," Tooth soothed him by stroking his hair and cradling him to her chest.

Jack thought about it and soon after, he contemplated if he really were strong enough to handle all of this. Indeed he was.

They say eye colors can change throughout childhood. One can start with a different color on his birth and then change into another color afterwards as he ages. I assumed this is true in Jack's case, where his eyes shifted from blue to brown (and back to blue when he became the Winter Spirit).