if that was a gift to anyone, it was a gift to rose, really, not the doctor...each individual element of the 'threefold man' was its own separate entity, and achieved its own sentience after being split off from the 'original'. (that's how i read it, anyway) therefore, the doctor himself isn't living a life with rose; a creature created from the union between him and donna is living that life, while the lonely god goes ever on alone in the universe.