The Doctor liked this fellow. He had an enquiring mind. He expected he would enjoy meeting young Obi-Wan, whenever in his future it was going to happen.
"Oh no," the Doctor answered. "You've got it precisely backwards. The history of your galaxy as I know it is precious because it is the product of the free will of the people who lived it."
Leela was following the Doctor and Obi-Wan on their walk through the desert. She was obviously bored and losing patience with the philosophical discussion. Fortunately Obi-Wan was intelligent as well as curious; the Doctor's errand here wouldn't take much longer.
"An ethical time-traveler," the Doctor continued, "such as myself, would protect that history just as you would act to keep an innocent man out of prison." Leela saw the bantha tracks at about the same time the Doctor heard the Sandpeople start calling to each other. They couldn't be seen yet, of course, because of a large dune blocking the line of sight.
Obi-Wan noticed all this too, of course, but it wasn't yet intrusive enuogh to rouse his curiosity; at least not given the chance to learn something new about the Universe. "What if the history you know isn't pure? What if there's been tampering you and the Time Lords don't know about?"
"That's exceedingly unlikely," the Doctor said. "History tends to seek its own level even without being watched over." Mentally he calculated that the Sandpeople should be just about arriving at the landspeeder now. "Of course, sometimes it takes a willed being as its agent, to give a nudge or two."
"For instance?" Obi-Wan was, now, being distracted by the noise the Sandpeople were making - noise he wouldn't have been in a position to notice, even with his Force-heightened senses, except for the Doctor's invitation to take a walk: The noise of Luke Skywalker having run afoul of Sandpeople on his chase after an astromech droid which was carrying a rather important message for Obi-Wan.
"For instance," said the Doctor, "I think you should be well-advised to find out what that racket is over this hill."
FIN