The Doctor adored travelling with his friends.
Rose had been his love of his life, but he regretted the day he lost her to the void. He also kicked himself for not saying that he loved her, he had so many chances.
Martha had been a nice distraction, she was friendly, and brave, but she had been infatuated, and he lied to her, shown off, plus had ruined hers and her family's life.
But Donna Noble, she was something special. Something brilliant.
Around Donna, he could be just himself, and she was fun. Always up for anything, and so captivated in anything he showed her, so animated. She was loud, fun, and willing to learn and he loved it. He couldn't put words to it, he just felt.. Comfortable with her.
He wouldn't wish any companion but her, especially after all the pain he had put himself and others through.