Memories of the past

After becoming a Guardian, Jack tried to pretend the memory of his past, and the 300 years of being alone never even happened, but the feeling of loneliness and guilt never left.

He put on a mask so much, he almost believed it was real. He was the Guardian of fun after all! No one ever left him alone about it either. Sure he pulled pranks, but that is only because he is a kid. He has been 15 for over 300 years.

Oh! Your still wondering what he felt guilty for? Well...he felt guilty for alot, feeling like he wasnt able to help the Guardians, Jamie slowly getting older and not believing in him anymore, feeling like he was a burden for the others, but mostly

Because of all the children his powers had killed

All of this together and living with the guardians, meaning he had the put on his mask even more, put alot of pressure on the youngest Guardian. You can't forget the bullies that almost made him pull off that mask too early.