This is a lesson that the show has been teaching us since very nearly the beginning (give in to your curiosity and you might accidentally get kidnapped by a mysterious alien time traveler!), but there's a much more recent and painful example. (Spoilers for Season 9, on the off chance you haven't watched it yet) Earlier in the season the Doctor used a young Viking girl to help ward off an alien invasion and because he made that choice she died. Out of guilt he used the available alien tech to bring her back to life, but this unfortunately made her immortal. Centuries later she inadvertently caused Clara's death. So, whose fault is Clara's death? Ashildr (Mayor Me)'s for luring them there? The Doctor's for making Ashildr immortal in the first place? Clara's for the choices she made that brought the curse on herself? All of the above? Ripples and Tidal waves, the Doctor said - ironically at the beginning of The Girl Who Died. Try to avoid the one and minimize to other, but there are always going to be consequences for the choices you make.