The Doctor's breathing was heavy, drinking her in with his eyes, he couldn't get enough. He thought he'd never see her again, and he was both overjoyed and overwhelmed by inexplicable sorrow. She'd asked him when he was from - his mouth closed tightly into a thin line, and he worried his brow. Should he tell her? Would it be too much for her to take? Was he even able to pretend that everything was alright. He glanced down at their hands together and realized that she deserved to know. If this was going to be his end, River of all people, ought to be warned.
This was the agitated and highly emotional state of mind he'd been experiencing in Knowhere, when these new memories came flooding in and motivated him to leave in the TARDIS without telling anybody where he was going or if he even was returning. Though he couldn't remember that anymore, something still stirred deep within. This time, he was going to stay and face his end, bravely.
"At the end of mine..." he mumbled, initially unable to look her in the eyes. "This is it. I've decided not to regenerate after this." Finally, he turned his face toward hers. "This really will be our last time together." A realization struck him. "Yes. That's what's going on. That's why it hasn't ended yet. I'm meant to see everybody more time. That's why Rose," his voice cracked upon speaking the name.
It was just like a couple of lifetimes ago - just before he regenerated, he'd taken the TARDIS to visit past companions and friends. He breathed out a chuckle when he realized, "Instead of me going to you, you've all come to see me off."