He had come to equate affection with sex in his time with the Master and he didn't see any problem with showing his appreciation for Owen's presence in this way. The Maser had always enjoyed it. Encouraged it even. The Master took care of him, and he gave the Master companionship and affection. It made sense to him. So he was surprised when Owen pushed him away.
"But I want to," he said, confused. He wanted to repay Owen for being there, for not leaving him. Why would Owen think he felt an obligation? He kissed Owen again, harder, clinging to him as if he were afraid Owen would disappear if he let go. The he would wake up and realise he was still alone.