Doctor Who Question

Something always kind of bugged me about Dr Who. First, I want to go on record that I LOVE the new Sci Fi Channel version. I liked the classic episodes, but they were always a little cheesey. Anyway, I have been watching some of the older shows on NetFlix and the new shows on SciFi. If you've never watch Dr Who, the Doctor (yes, that's his name) is a Time Lord that travels through space and time foiling the plots of evil doers. In the older version, there are a few other Time Lords and the Doctor was sort of an outcast/black sheep of the Time Lords. Still, they made him their bitch on a few occasions to do things that they couldn't. There was also a bad Time Lord know as The Master (not big on names these Time Lords). In the Newer version, The Doctor is the only survivor of the Time Lords after the Last Time War against the Daleks (evil cyborg killing machines). OK, to my question: If The Doctor can travel through time and space to foil plots of evil doers, why can't he go back and save some of the other Time Lords? I guess that the arguement is that he cannot go back and fix things that have already happened or else it could have negative effects on the present/future, but that's what he does. He goes back and forth in time fixing things and stopping the bad guys. You could say, "Well, he's stopping the bad guys." but what if time dictates that the bad guys were supposed to win? He's altering the present/future to suit what he feels is the correct route. If he goes back in time to stop the Cybermen from dominating the galaxy, isn't he having a negative effect on the time line of the Cybermen? I really like the show, and my question/concerns are not going to discourage me from enjoying the next episode. It's just something that sometimes pops into my head from time to time.