John is nothing if not competent, thriving when he is needed. He resents it. After Sherlock's death he falls back into the depression Sherlock saved him from after Afghanistan. Then Sherlock returns amid dead torturers and John's clinical depression and the only real communication Sherlock seems to be able to have is of the very physical kind.

They never talk about it, Sherlock and him. It just happens, hidden by the night and created by the hatred they feel for each other. Morning will be normal again.