The reason the Doctor does not normally try to go back in time to prevent disasters is apparently because those kinds of changes would send shockwaves throughout the universe. You cant alter one thing without it having a domino effect on everything else that happens. The TARDIS knows this and acts to prevent the Doctor from making that mistake. The Doctor knows this also but hes willing to break the rules in order to save his friend. Once again we have allusions to The Waters of Mars episode and the Time Lord victorious. You might have the power to play god but that doesnt mean you should. Time has a way of righting itself when anyone attempts to muck about with it. The Doctor acknowledges this by explaining if you lose someone and go back in time to prevent that all you end up with is conversations with a ghost. Bennetts accusations against the Doctor for allowing ODonnell to die in order to test a theory echo back to last series Mummy On The Orient Express. The Doctor is willing to sacrifice lives in order to solve the problem but he isnt so sanguine when the consequences hit closer to home.