Over the many years of the Doctor's life he learned many, many different lessons. He has learned that curiosity counts and being weird is cool. He learned that life is complicated and people could be scary. However, he also learned people and life are also could be amazing. He learned that home is where your feet are, to always embrace new experiences, and never ever take anything for granted. But, most importantly you can always reinvent yourself.
The doctor at this point in time has reinvented himself fifteen times. Three more times then what is recommend for a time lord. But, there is no one else in world that could take his place and without him the universe would fall. The dilemma is what caused him to learn the hardest lesson he learned he ever learned. The lesson he has learned over and over again, and that is love doesn't conquer all. It was sad, but its truth.
Every lesson helped make the doctor who is. However that last lesson is destroying him on the inside. But, what do you expect from a man who had witnessed countless people die in his name and had to say goodbye to anyone he loved. Its no wonder why is currently alone in the TARDIS, and has been alone for months. He hates to be alone and he knows he is not his best when he is alone. But, he hates the goodbyes even more. People would think his two hearts would make him stronger, but there is only so much they take.
And with every goodbye both of them break. Nerveless, there is a light at the end of tunnel. The Doctors blood, sweat, and tears will finally pay off. The universe will finally reward him for all that is has done all these years. He will soon come to the point in his life where he will never be alone in the TARDIS ever again. He just has to wait a little bit longer…..