I last waxed poetic on the constant theme of acceptance woven throughout the series, now I would like to point out another thread. This thread weaves a dark subtext but one every being no matter what their views has experienced.
The Doctor most times is joyful, full of discovery, defending the oppressed, basically saving the universe and the Earth repeatedly.
Sometimes this bitterness is subtly touched upon and others times its thrown at the viewer with brutal force.
This sharp blade is loneliness; the realization that YOU are the one that is different, and no matter how much you strive this can never change.
In his extensive life with all his vast knowledge The Doctor knows that sooner or later he will again be alone. As the only one of his kind he can never hope to find that constant companion that could turn into a full time partner.
What individual has not faced this pit? It matters not if you were or are the perfect social butterfly, beloved and respected by your peers. You don’t have to be the individual struggling with your sexuality, discovering and processing your views on life and injustice. You can be absolutely the vanilla definition of normal and still be struck down by the thought that no one can ever understand you and you face the onslaught of life alone
This is why he searches and collects his companions but never allows himself to be too close. Of course the subject of love and attachment was explored with the companion Rose. Even this was fleeting and ended, changing his attitude and outlook even more with regards to emotional connections.
The current Doctor, Matt Smith, of course is not near as serious as The Doctor portrayed by Tenant, but his first companion is essentially already taken and attached when she joins him in his adventures.
The Doctor teaches us that even if we are different and exist in an environment where no one is fully able to relate to our thoughts and feelings; there is still joy to be found. You can still strive to find happiness and teach those around you to understand your views, if not help them to accept and embrace them.
To place this subtext in the real world, do we not strive to teach others that all genders, race, religion, beliefs and sexual orientation are something to be embraced and respected? Learning about these views and accepting them is not something evil but a new adventure to be explored. Is there anyone that at one time was the only individual in a group with a different slant or view of importance that others couldn’t understand? Thus leaving us “ALONE” with our outlook and misunderstood?
The Doctor shows us these feelings and situations can be dealt with and overcome. Although we may be alone we can persevere and make a difference. We may have too,time and again, face the pain of loss and return to being on our own. This being said The Doctor shows us you should never give up and sequester yourself away from others. With belief and knowledge you can open eyes and set others on a path to explore the universe.