While he really should have seen the kiss coming as she moved closer to him, the Doctor was so caught up in her words and his own emotions to really think clearly, so when her lips met his he stayed quite still for a second as if in complete shock. After the initial pause, however, he returned it as if her lips were providing him with life.
He gasped a little as she pulled away again, looking into her eyes with a mixture of amazement and affection. It took him a moment to respond but when he'd managed to compose himself again he gave his head a sharp shake side-to-side, dismissing the apology. "No. That's... quite all right!" he told her, a little high-pitched as a small smile played on his lips. With a kiss like that, there was no way this was a hallucination.