Rose Tyler can not be discounted in this either. Eccleston had traveled throughout our history for a considerable period of time - based on images of him Rose found in her research. As I noted earlier, I suspect this was about cleaning up the aftermath of the Time War - things like the Zygons and the Autons.
But in all that time, and perhaps throughout the entire Time War, the Doctor had been alone. Rose was his first Companion in centuries. Having her along to share his adventures, and to give him a reason to visit more pleasant places, allowed him to set aside the pain, guilt, and sorrow, and live again.
Tennant, "her Doctor", was shaped by her influence during his previous life. Much like Smith was shaped by his encounter with young Amelia.