The Doctor could taste himself on Jack's mouth, and that was quite the seductive sensation in and of itself. He kissed back as though it were breathing. In a way, it felt like it was. His energy went into Jack who then fed it back into the Doctor. Like respiration. Well, a bit more like chemosymbiosis, to be honest. Jack took the sulfides the Doctor couldn't process and made them into workable, usable sugars. The Doctor, in turn, shared the energy with Jack and with the help of the TARDIS, who the Doctor supposed could be photosynthetic bacteria in this rather extravagant metaphor---

Nah, he didn't need metaphors. This was good as it was.

"You know," the Doctor said, grinning. "I don't really think I'd mind that on a regular basis, no."

My turn.