He's going to get her killed. The way he killed Irene. He still remembers the letter, the one meant for him, that showed how M was following him, had targeted him and the woman he loved. What would happen now if the people following him knew how much he cared for Watson?

Selfish. He's selfish.

When he breaks the kiss this time, he moves his mouth to that place on her jaw he was examining before, and traces it down her throat.