Hober Mallow wrote:
Here's my idea for a retconned explanation:
The Doctor's experiences with Susan and Ian changed him utterly. First, we have Susan's love of 20th Century earth, a love he would adopt partly because of her, and party because of Ian. In time, he comes to admire Ian's spirit of adventure, Ian's taking of moral positions and putting them into action, his getting involved, and that this, more than physical regeneration, gave the Doctor a renewed spirit. Eventually, even the Gallifreyans would take notice of the Doc's newfound obsession with earth, especially earth of the 20th Century, as they mention to Patrick Troughton's Doctor before they force his regeneration and institue his exile on earth.
That's pretty much the consensus, and something that later writers have taken up and run with; Russell T Davies did a sort of condensed version of the early Hartnell character arc in the Eccleston season, with Rose Tyler meeting a damaged, scarred Doctor and changing him for the better. I wouldn't even call that theory a retcon - it seems to be what was presented on screen at the time. After all, the first Doctor changes radically and becomes much closer to the more familiar character, even before the first regeneration.
Also, a tiny point; it's Davison, not Davidson.