He liked hearing Jack giggle. He was so uninhibited, and not just in the obvious way - he showed his reactions, nice and straightforward.
The Doctor ran his finger up Jack's back again and said airly, "Oh, it didn't take much, as I recall. Little bit of dancing - taking it in turns with Rose; little bit more dancing after Rose had gone to bed; and then things just sort of... happened."
He smirked, leaned in for a quick kiss, then turned away again, towards the house, still holding Jack's hand. "Come on!"