My theory:
In a hyperthetical story called "The Executioners Song", the Doctor finds himself back in the Time War. Thought to be time locked and irreversible, the Doc is once again fighting Daleks and watching Gallifrey burn. Suddenly the Doctor is killed by a Dalek but he is incinerated. The Woman from "The End of Time" gathers up his ashes and takes them to a mystical Gallifreyan witch Doctor. The witch doctor puts the ashes onto a special plinth and says some cryptic words. The End of Time woman is crying and pleading with the witch doctor to restore him.
Suddenly there is a glimmer of hope as the ashes form into a snake like creature. The creature becomes powerful and the Woman bellows "A time lords true form". "Behold the resurrection of the Time Lord". The creature gets bigger and suddenly forms into a gigantic white light. The white light dims an a figure of a humanoid male stands before the woman and the Witch Doctor. "Hello mother" says the figure. "Hello son" says the woman. They hug each other.
The figure looks over to the witch doctor. "Am I restored?" he asks. The witch Doctor nods. "You are immortal but only until you get to your thirteenth ressurrection. After that, you will die."
"13 more regenerations?" asked the figure (who we know as The Doctor.) "Yes. 13 more. After that you must seek assistance from your mightiest enemy for further lives."
"What do you mean?" asked the Doctor.
"Those who you fear, those who would be your executioner. It is they you must sing to should further restorations be necessary. It is they, and they alone who can grant you new further cycles. Fail to gain their assistance and you will die forever. Your Tardis will be your guide".
"My Tardis. My guide. Just as she's always been!" says the Doctor. The woman looks lovingly at the Doctor. Tears are forming in her eyes.
"Now go, my son. You have a life refreshed, 13 new lives to live. Your first act must be the hardest act you will ever undertake."
The doctor, looking concerned, but knowing his mother's implications, kissed her on her cheeks tenderly.
"We do not belong. We have no place in the universe anymore. You and you alone my son, must restore time back to before your death. The time War must be locked. The Time war must be won by no one. Remember the Witch doctors wisdom my son. Protect those you fear the most. The universe may, some day, depend on them. "
The Doctor held his mother tight one final time and just as he had thousands of times before, relied on his dear old faithful Tardis to guide him out of the Time war and he successfully locked it again. a brand new Doctor. a brand new cycle of regenerations. brand new adventures!