Humans are such horribly short-lived creatures, at least they are from a Time Lord’s point of view. In the right circumstances, providing he doesn’t put himself in too much unnecessary danger, each of the Doctor’s regenerations could last him hundreds of years. He could, in theory, live for millennia while the average human is lucky to survive much past eighty. Even then, by that point their health will most likely be failing, their bodies growing weaker, their memory increasingly unreliable, their senses, never as sharp as a Time Lord’s in the first place, losing clarity, poor eyesight and hearing distancing them from the world they live in. They’re as ephemeral in their way as mayflies; here today then gone in the blink of an eye.
That’s one of the reasons the Doctor eventually chooses to leave his human companions behind, never keeping any of them with him for more than a few brief years. Watching them age and fade away would be too painful; he has no desire to watch such vibrant, amazing people, his dear friends, wither and die.
Some might wonder why he even bothers to have human companions when there are so many longer-lived and less fragile races in the universe to choose from. But despite their short existences, or perhaps because they’re so painfully aware of how little time they have ahead of them, humans burn brighter than anyone else. They live more fully, feel more intensely, display more curiosity and fascination with the unknown than any of the more enduring races. They’re a young race and they’re driven by an irrepressible desire to learn and to discover everything that exists beyond their own tiny planet.
To humans the universe is a place of infinite wonder; there’s so much to see and because so few of them ever get to leave the world of their birth those that do throw themselves wholeheartedly into the adventure. They’re like sponges, wanting to absorb as much as possible, experience everything they can, meet other peoples, learn about other cultures… No one else the Doctor has travelled with has ever shown more enthusiasm for exploration, more excitement over what to most races would seem very ordinary places and things.
Through his human companions, the Doctor gets to see and experience everything anew. He gets to be a teacher, instructing people who have never met aliens before on how to interact with beings so different from themselves as to be almost incomprehensible. Perhaps, by the time humanity takes to the stars, travelling between worlds and setting up colonies on distant planets, enough of what he’s teaching his companions will have filtered into the collective unconscious that they will encounter fewer problems than they otherwise might have. At least he can always say he tried his best to educate them.
It would be easy for someone whose lifespan may measure thousands of years to become jaded: been there, done that, seen it all before. But humans provide him with new, unexpected perspectives; they’re a delightful breath of fresh air, eager, unspoiled children with a zest for life that’s infectious, and… he loves them. They’re far from perfect with all their prejudices, fears, phobias, and naïveté, but they fascinate him, draw him to them like iron to a magnet.
He’s painfully aware, however, that they’re the moths to his flame. They’re as drawn to him as he is to them, yearning for adventure, longing for excitement to brighten what they see as their dull, ordinary, uninteresting lives. Not all humans of course, but the ones who have what it takes to travel among the stars. The adventurous ones, the dreamers, and rebels, and stargazers. He can show them so much, things they’ve never dreamed of, but travelling with him is not without risk. Always, at the back of his mind, is the knowledge that he could be taking them to their deaths; it’s happened before, and the possibility breaks his hearts. He protects his companions to the best of his ability, would give his life and all his regenerations for them, but things can happen that even he is unable to foresee or alter.
So in the end, usually long before they’re ready to give up travelling, he sets them back down where he found them and leaves them behind to pick up the pieces of their old lives and try to find satisfaction in raising a family, or having a career, when their hearts are still out there with him in the black of space, discovering alien worlds, and travelling through time.
It hurts him as much as it hurts them, he always wishes they could stay with him forever, but he knows that’s impossible. So just like all his past companions he has to content himself with memories of their time together. Better that they be safe and alive back on earth than that their fragile lives should be snuffed out far too soon through his carelessness, their overconfidence, or the fickle whims of fate.
It’s small consolation for those left behind, and he knows many of them resent him for giving them so much only to take it all away again, but they don’t have forever, and he won’t cheat the earth of the contributions they might make in their lives to come.
Who knows, perhaps without being consciously aware of it he’s shaping earth’s destiny, one person at a time.