This is one of my favorite scenes on Doctor Who because it describes so perfectly the Doctor’s relationship with humanity. He lives in no time and all time at the same time.
In a way, everyone really is a ghost to him. It’s almost a perfect description — anyone can be dead or alive to him at any moment. He can whisk a young woman off to another time where her body has long been buried six feet under the ground.
Clara is dead according to the calendar, but also alive and standing next to him. Rose Tyler is trapped in another world, but he can go help her with her homework. He can witness River’s Song’s death not knowing who she is and still fall in love with her and marry her later.
Life, death, and time are completely tied together in a linear fashion to us, but they aren’t to The Doctor.
And so he can treat the end of the Earth like simply another day while it brings Clara to tears. To him, the end of anything is always there. It can always be known and even experienced, but he can usually go back to the middle. The end of Earth is certainly such a case. The end of a world is simply another time to visit. He’s even already seen this moment from a different angle. As much as The Doctor loves humanity, he is not human and Earth is not his home.
But to Clara, the end of Earth is a far off and terrifying concept. Even when introduced to all of time and space, Earth is her home. It’s everything that means anything to her. Endings are so very final and, like Rose Tyler before her and nearly anyone born and raised on Earth, she would not have the perspective handle the witnessing inevitability of the end of her world without it deeply effecting her.
this was never about winning. it’s just about surviving. the machine and i couldn’t save the world. we had to settle for protecting the seven people who might be able to take it back. so we gave samaritan a blind spot: seven key servers that hard codes it to ignore seven carefully crafted new identities. when the whole world is watched, filed, indexed, numbered, the only way to disappear is to appear. hiding our true identities inside a seemingly ordinary life. you’re not a free man anymore, harold. you’re just a number. we have to become these people now. and if we don’t, they’ll find us, and they’ll kill us. i’m sorry, harold. i know it’s not enough. a lot of people are gonna die. people who might have been able to help. everything is changing. i don’t know if it will ever get better. but it’s going to get worse. the machine asked me to tell you something before we part - you once told john the whole point of pandora’s box is that once you’ve opened it, you can’t close it again. she wanted me to remind you of how the story ends. when everything is over, when the worst has happened…there’s still one thing left in pandora’s box: hope.