Drake had been worried when Sylvie wasn't home when he expected her to be. She was always there when he got back from work. Then his first call didn't go through. That made him even more worried.
He had been relieved when she came home until she suddenly threw knowing the truth into his face. How?! He had been so careful!
He was horrified but still had to come up with something to say—anything that would prevent her from leaving him. He didn't care about being used at this point. He was hers. He would do anything for her. But he wouldn't be able to stand being left behind when he finally knew what it was like to experience love.
But her reaction wasn't what he expected. She was confused rather than afraid. When he asked why, she said that he was the only person who ever really cared about her and that she just wanted to understand.