Look critically at the two men; the heart and the head.
The heart: Sherlock. Selfish and undying love, adoration, extreme sacrifice, protection at all costs, can only see the man he loves as the perfect man, looks at no other, lives his life for John. His avowed priority, the work, is thrown to the wind for the man he loves. Look at him, Sherlock is all heart.
The head: John. Cannot allow himself to voice his emotions. Blind to the love emanating from Sherlock as he has convinced himself that Sherlock can not love. Forces himself to comply with rules and expectations, forces himself to ‘move on’, stands by his decisions even in the face of incredible unfolding events. Takes a logical approach to his choices in life. Lives a different life to the life he really wants with Sherlock.
John is so in his own head, so overthinking things, that he cannot see what is right before his eyes. He is bogged down by conformity to the rules he has set himself, probably from standards set by parents. He chose the army as a career; rules, regulations, order, hierarchy. Now in marriage and his GP career John is killing himself slowly, living a life that is not for him. His head is ruling his fate. Whereas Sherlock has abandoned all logic to run at full pace into love. The head and the heart, but not as we initialy thought.