"I was there in the great time war"
The Doctor stood, staring out of the TARDIS window at the remains of Gallifraey, his home planet. He was the sole survivor in a war for power. A war that he alone had no choice but ended. He was alone now, he could never love, he could never settle down, and no one else could ever understand what he had done.
"My freedom came with a price"
The Doctor can only ever walk alone, everybody he ever travels with, everyone who follows him to the end- they never live long enough to reach their own true ending. They never live long enough to understand that a normal life is one that the Doctor would love, for he does not understand his companions lust for adventure. He does not understand the desire to follow him, he doesn't believe himself to be all that special, just a parasite to the human race.
"I oversaw the destruction of two races- one of them my own"
Together the Time Lords and the Dalek races fell into oblivion. Some say the Doctor had no choice, that he had to save time, and the rest of the universe. One look at the Doctor will show sorrow at all that has previously befallen him. It is sometimes rumoured that he saw the fall of Arkadia, and even fought on the front lines. Is this true? Nobody knows, nobody will ever know
"I executed the plan. I did what I had to do"