Time Lords were a mysterious race with mysterious capabilities. The most well-known of these capabilities is the ability to regenerate after a mortal injury. The regeneration process was very complicated and very painful however the physical pain was not what worried the Doctor this time around; after all, he had already gone through the process 9 times before.
In all his time of life he had come to know many races from many different planets and times, however humans always held a special place in his hearts. Almost all of the companions he had travelled with had been human and he had gotten a strong connection with all of them. While there were some who left the doctor in a better state than they had started, it was not always the case. The Doctor had lost many of them and, most recently, left one in a constant near-death state.
His companions weren't the only ones to lose their lives however. No matter where the doctor went death was sure to accompany him. For over 900 years the Doctor had had to deal with the deaths of many people and this fact weighed on him.
While earlier in his life he allowed himself to be slightly bitter he refused to let it show now. The tenth incarnation of himself was known for being excitable and impulsive but most of all happy. The personality that came with the new body allowed him to repress the sadness that had been growing in him over the years.
After he took the full blow of radiation while stopping the master once again one thing became completely clear to the doctor: he would no longer be able to ignore his heavy hearts any more. No matter what persona he would take on next he knew that he would not be lucky enough to remain happy for another lifetime; he would finally have to all the pain he had caused.
He needed to know that he had left his friends with some happiness, so he decided to see them all one last time and really help them. He saved Sarah Jane's son from a car accident, set Jack up with a respectable man, given the Noble family a ticket out of poverty. He saved his favourite idiot and his wife from death at the hands of a Sontaran and even saw the girl he had come to love one last time and yet it was never enough. They had all looked at him with pity, he was dying and they knew it; even when bringing them out of danger instead of towards it he still left them feeling unhappy.
When the time finally came for him to leave the universe and a new Doctor to replace him, he could feel the depression rising to the surface. He could sense himself becoming more recluse and new his suspicions were correct. In one last plea to whoever could hear him he begged not to die, for mercy, but it never came. The energy took over, destroying his last hopes and the Tardis with them in a flash of blinding light.
Of course he had tried to hang on for a little longer, the Doctor was a fighter after all. He allowed himself to bring along two more companions on his adventures but history repeated itself once again. In almost no time at all the Pond's saw more death and pain than any other companion before and were soon met with the swift departure from the life they knew at the hands of an Angel. Once again the Doctor had lost those he held dearest and it was the final straw for him. For the next 200 years he remained detached from the universe and vowed to never again harm anyone else.
However even for someone who has seen all of time and space, there are still surprises to be had. This particular surprise seemed to keep cropping up in the Doctor's life; this Clara "Oswin" Oswald had appeared multiple times now in the Doctor's life and maybe she was the key to a better life for him. It seemed unlikely at first, she had appeared twice before and he had failed to save her both times, but stranger things had happened to the Doctor. Against his better judgement, he left the safety of solitude and made a house call to Clara. Within moments of meeting this Clara the Doctor knew she was sent to help him. Not only did she bring him out of his loneliness, she gave him something other than death to focus on. This girl was almost a bigger mystery than the Doctor was and he could not allow that. From that day forward he dedicated his life to both save everyone he possibly could and to keep Clara alive while discovering the hidden past surrounding the impossible girl.
Like the Doctor had hoped his newest companion turned out to be different than the rest. She brought true happiness back into his life and allowed him to see the good in the universe. Even when all had seemed lost at the end he never gave up or lost hope in her. Rather than submit to defeat when she seemed most lost he rose up and fought back; refusing to lose anyone else. Clara did not know it but in her attempt to save the Doctor from the Great Intelligence, she also saved him from himself.