Malcolm gapes at Sherlock, actually speechless for a second at such an accusation. "Being careless with my life is one thing, but I am never careless with the lives of anyone else, particularly not Joan." He loves her and would do anything to protect her. He'd jump in front of a bullet for her.

"You're jealous of our relationship," Malcolm shoots back, unable to stop himself. "You always have been. Oddly enough, it's not because you want to sleep with her. It's because you feel like she's the only one who understands you and you're afraid of losing her."

Ironically, Malcolm feels like Joan is the only one who understands him too.