The Doctor’s past is filled with so much pain and heartbreak, more perhaps than any other living being has ever known. Countless friends and loved ones have passed beyond the veil over the centuries, leaving him to carry on alone. Companions have come along to brighten his existence for a brief period of time, but they can never stay for long. They age too quickly, and he has to let them go before his way of life can end theirs.
The last of his race, he feels compelled to keep moving forward, because looking back will only break his hearts.