It must be because he's so tired.

That is the only explanation for why Sherlock is not analyzing this moment on every available level, taking it apart at the seams to understand just what is going on and why and how it happened. He has in fact stopped at the level of awareness that says this is entirely perfect, and is reaching no farther.

But he doesn't feel tired anymore. The weight of exhaustion lifted around the time Cal met his eyes. He just feels right. As though this of all things is exactly what he needed.

His own hand rises to cover Cal's.