Severus doesn't stop looking at Remus, as his lover repeats his words and the intention and feelings behind them as well.

The kiss seals the words deeply inside Severus, turning this moment into much more.

"You shouldn't worry, Remus." More importantly, he doesn't want Remus to worry. There's no way around going back. But what his lover says is actually true. He does take a part of Remus with him, every time he leaves. Severus also leaves part of himself behind, keeping it safe from the corruption and filth he's surrounded by when at his ... Lord's side. And knowing it will be there for him, when he returns, makes it bearable.

"I do," he says truthfully, his hand still cupping Remus' face. "And it keeps me sane." Or as sane as possible.