During almost the entirety of Classic Who, the Doctor was on the run from his home planet, Gallifrey. He had wanted to do things his way and would not conform to Gallifreyan law and customs, instead going "on the lam" and finding a way out, albeit whilst getting himself captured by the Gallifreyans on a few occasions, When NuWho arrived, viewers learned that Gallifrey, as well as all the Time Lords, had been destroyed in something called The Last Great Time War. This caused the Doctor to be incredibly depressed, as not only was he the one who caused their destruction (or so he thought), but he was also, to his knowledge, the only remaining Time Lord in existence. For all the time he spent running away from them, he wanted nothing more than to be able to run to them at this guilt-ridden point in his life. Lesson Learned: Don't take anything for granted because you'll miss it that much more when it's gone forever.