As the regeneration energy started to pulse in a golden light out of his hands, the Doctor eyes teared up, and an anguished expression filled his face.
"I don't want to go," he cried.
He didn't want to go, because then he knows he will never see Rose again. And that's exactly why he had to die – he'd sent her off to someone else, his lucky doppelganger, and now his own chances of ever being happy with her are gone – there's nothing left for this regeneration to live for. Even when she was gone, before she'd come back, he'd had hope. He didn't try to get back to her – he didn't dare try, he knew it was supposed to be impossible. Nonetheless, he'd waited for her. He waited to see if, as always, something wonderful and impossible would just happen. He should have known that she would not be so passive. She'd once torn open the TARDIS with a tow truck to get back to him… why should a parallel universe stop her? But now, it really was impossible, not because the parallel was closed off, but because he'd chosen not to say those three words, had let the doppelganger take his place, because he knew it was better for her. And now he could never go back, because it would just confuse her and ruin his gift to her – a happy life. And that is why he had to die… he had no hope left. Even this chance to see her one last time, to see her beautiful smile, was both a wonderful gift and a horrible curse – because no matter how warmly she greeted him, almost like an old friend, she still didn't know him, he still couldn't open his arms and wait for her to fall into them like she had so many times before, and that, that, really killed him.