Ever since childhood, the Doctor has had a yen to travel, to see things others of his race never have and never will.
Most Gallifreyans, even most Time Lords, are happy to remain on their own world, but not him. He’s never been satisfied with such a limited existence, not when there’s a whole universe and all of time waiting to be explored.
It’s like an itch, always there, urging him on over the next hill, or to the next world. Then when he’s seen what there is to see, he’s ready to move on again.