The first time he met Rose Tyler, she shook him to the very core. She was everything he needed but didn't want. He was old and worn and bitter; she was young and fresh and so very alive. But he took her along, anyways.
The second time Rose Tyler met him, he was bursting with energy, and a strange new emotion seemed to overload his system. Love, he finally realized. It wasn't as though he didn't love her before. He did. Oh, he did. But this was so much more powerful and intrinsic and raw; he was born for Rose Tyler.
He'd loved all of his companions; how couldn't he? They were all brilliant and charismatic in their own right. But Rose Tyler had always been the exception, the irregular orbit to his systematic, precise way of living. With every smile, every touch of their hands, he felt himself slipping further and further away; he felt himself being drawn closer and closer to her.
He was the planet and she was the sun, drawing him in and in until his controlled way of living was completely upside-down, and he centered himself around her, around them.
Now that she was gone, he was stuck in this new orbit, and without her to ground him, he didn't know how to get himself back on track.
So he burnt up a sun just to see her one last time, knowing that he was throwing an entire solar system into a new, irregular orbit. He left them floundering, struggling to understand their new, off-kilter lives.
He thought it seemed rather appropriate.