While I agree with nshapero that obviously the Doctor changes time in SOME way every time he travels, what caused this change to be a damaging one was the paradoxes - two Roses and two Doctors in the beginning and then Rose holding Baby Rose. The Doctor said with the Tardis he could fix the problem and leave Pete alive - until Rose held Baby Rose - that paradox was too much to fix. Thus, Pete had to die to return circumstances to their original point.
There is some cheesy dialog and forcible tear jerking in this script - they do squeeze the emotion for all it's worth - but it's worth it. To see Pete slowly figure out what was happening and why. To knock that "this is just a great adventure" attitude out of Rose and make her grow up a little, to see her parents' marriage wasn't a perfect love story but that they were human with human flaws, even Mickey gravitating straight to Rose the first time he sees her, each moment had its effect and its meaning.
And then there's the repeated idea that Pete's life and career would never have taken him anywhere even if he had lived.... but, well, that's another story....
There is some cheesy dialog and forcible tear jerking in this script - they do squeeze the emotion for all it's worth - but it's worth it. To see Pete slowly figure out what was happening and why. To knock that "this is just a great adventure" attitude out of Rose and make her grow up a little, to see her parents' marriage wasn't a perfect love story but that they were human with human flaws, even Mickey gravitating straight to Rose the first time he sees her, each moment had its effect and its meaning.
And then there's the repeated idea that Pete's life and career would never have taken him anywhere even if he had lived.... but, well, that's another story....