Tsuna always knew that he was not real. After all he was only the remains of the real Tsuna. He was left after Tsuna died because of the seal that was placed on him. Tsuna always wanted to live have friends that care about him for who he is.
His wise was granted for him when Reborn came to his home. Even though he was hurt at times sad at times he never felt so alive. That was why he wanted to give them something in return. Originally, he was only supposed to exist until Tsuna's fifteenth birthday, but when the seal was removed he was given more time as the remaining flame that he has was the only thing that was keeping him 'alive'.
The more flame that he uses the less time he has. He didn't care about that. The most important thing is, was his friend family and everything important to him. When they went to the future and when he created x-burner he knew that they will shorten his time.
He kept fighting even if it will kill him. Primo may knew about it since the beginning since he would always look at him with sadness in his eyes. The sadness that no one know the meaning or the reason behind it.
When the battle of the past happened. Tsuna found out that deamon found out his secret. That was the reason of why he didn't want Tsuna to became boss. Tsuna was happy even with the hardship that he had.
The battle of the rainbow was the last thing that decided his fate and time. When he found out what the arcobaleno was and what the battle was really about. He decided that he will do all he can do to save to save Reborn.
Even if becoming arcobaleno might save him. He would save his most important people from the fate that will save him. He really wanted to spend more time with them but his time has become little as he only has about until his eighteenth birthday.
Time past and Tsuna did everything that he want to do with his friends. He had so much fun. He also got to punch his 'Father' that one time. He got a really dark black eye. If there was something that Tsuna regret was not being able to spend more time with them.
Tsuna didn't know what to say. At the party he started glowing after saying the thing that he wanted to say the most.
"thank you." Thank you for being there for me. Thank you for accepting me. Thank you for loving and liking me. Thank you for helping me. Thank you for being a wonderful friend. Thank you for being the family that I ever longed for.
The meaning behind his words were clear. The only thing that was clouded was what happing to him.
"Tsuna what do you mean? and why are you glowing?" with reborn using his name not Dame-Tsuna show how series he is and that he alo don't know what is happening.
"All I can say is I'm happy to meet all of you. I wise that I would be able to spend more time with everyone. Thank you for everything, meeting all of you is the best thing that happened to me. Goodbye." With every word he said some part of his body start turning into little balls of light, with tears running down his face. The moment he mouthed 'Goodbye', he completely disappeared.
All they could do was watching him disappear in front of their eyes while not being able to do anything. They later found what happened from Tsuna's journal. They could not feel any better with what happen. Reborn for the first time felt that he failed.