As a wrinkled and decrepit Clara Oswald once said, every Christmas should be treated as last Christmas. Any moment could be someone's last. The Doctor is well aware of this dismal reality after losing so many companions in a tragic clash of fates... and with more still yet to come (looking at you, impossible girl). We sometimes act as though time travel were possible, that time won't be cruel or merciless once it expires, and that the clock can be recalibrated and reset. Time we share with others is taken for granted, and then, all too soon, people are gone, like breath on a mirror. Even the Doctor knows that time travelling has its limits and that some goodbyes are destined to last forever.