Disorder(s): Somatoform disorder (later disappeared), possible form of post-traumatic stress disorder
Positive traits: Strong ability to empathize with others, strong ability to uphold commitments, well-developed sense of loyalty and desire to protect loved ones, heightened sense of morality/justice, selfless, strong ability to keep calm in the face of danger
Negative traits: Tendency to bottle up anger until it is released in outbursts, dislike of receiving pity or sympathy, tendency to exhibit reckless and/or dangerous behavior under the guise of bravery or justice, suspicion/mistrust of the intentions of others
Notes: John Watson is mostly mentally stable, although he does have his fair share of negative and positive traits. One of the most interesting, in my opinion, is his desire to put himself in stressful or risky situations in order to gain the pleasure of excitement.
After John Watson was discharged and sent back to England from Afghanistan, he subconsciously felt inadequate or crippled; as if he had been stripped of his purpose. These feelings, combined with the physical trauma he experienced, may have manifested itself into what became his psychosomatic limp (somatoform disorder). His feelings of being mentally or emotionally crippled seemed to have digressed into a physical sensation. All of this may also be characteristics of a form of post-traumatic stress disorder.
People who are relatively calm and patient by nature often bottle up their anger. This anger is usually only released by a certain, very specific trigger. In John’s case, it appears to be several things, ranging from his limp to, of course, Sherlock Holmes (A Study in Pink: John yells “DAMN MY LEG!”, The Reichenbach Fall: when the Chief Inspector insulted Sherlock, John punches him in the face, In A Scandal in Belgravia, when Sherlock was supposedly heartbroken by Irene Adler’s “death”, John threatened her without hesitation when he found out that she was alive). All of this is also evidence of John Watson’s protective tendencies; he is very selfless and clearly very protective of those who are close to him (I don’t need to mention John shooting the cabbie for Sherlock or grabbing Moriarty in the swimming pool, do I?). John also appears to have the very useful quality of being able to stay calm and collected in dangerous situations, as well as of being very brave; qualities that have a lot to do with him previously being a soldier, but also to do with his characteristics as a human being.
As with Sherlock and John’s relationship, I think it transcends regular friendship in the way that they really do, in fact, need each other. Sherlock makes John feel whole; he feels alive, he feels needed when he is with Sherlock (“When you walk with Sherlock Holmes, you see the battlefield), also as evidenced by the disappearance of his limp. Sherlock needs John not only to have someone around that admires his abilities and supports him, but will also simultaneously keep him grounded and warn him when he goes too far; someone who’s very presence may be able prevent him from doing things that could be harmful to himself or others.