Dominant Si: John has a difficult time getting over the past. Two years after Sherlock’s death, he is still grieving, and even after years away from Afghanistan, he still thinks of himself as the same army doctor. He is initially drawn to Sherlock because being with him brings back experiences from the war. You could almost say his past has made him who he is today… He needs evidence to believe, which is why he continues to ask Sherlock to explain his deductions even after living with him for ages. Along the evidence line, when he first meets Sherlock, he doesn’t believe the police or Mycroft when they tell him to stay away from Sherlock. Instead, he decides to find out what the man is like for himself. John holds with society’s traditions, but his adventurous side doesn’t always enjoy them. He plans a wedding, but would rather be solving crimes with Sherlock than testing cake (my theory is that Mary set that little outing up for both of them, not just Sherlock). John prefers life to be predictable, and he gets very frustrated whenever Sherlock goes off on his own without communicating or when Sherlock asks him to do something that he is too lazy to do himself.
Auxiliary Fe: John is aware of and considerate of other people’s feelings and does what he can to clean up the damage left behind by Sherlock’s blunt communication style. John doesn’t always understand his own feelings very well, and tends to detach from them whenever he can. At times, he can burst out in anger, but despite all of Sherlock’s shortcomings, he doesn’t abandon the friendship. He never ceases to put aside his own needs and feelings to help Sherlock. He sees what is most important to other people and does everything he can to support them (so long as it doesn’t hurt anyone else). He doesn’t know how or want to talk about his own feelings, though he desperately needs to, and because he often doesn’t, it becomes even more impossible for him to move past things. He has a therapist, but he doesn’t usually tell her as much as he needs to. He writes a blog because it supposedly helps him cope. Another reason that he has trouble moving beyond the past is his difficulty understanding his emotions. He is frequently used and hurt by others, but doesn’t always know how to cope with this, because he doesn’t know how he feels about it.
Tertiary Ti: John is a critical thinker in a much different way than Sherlock. He’s quick to figure out what isn’t working in a situation and eliminate it. He wants things to run smoothly and doesn’t particularly appreciate it when Sherlock goes off on his own with no explanation. John gets annoyed whenever Sherlock shows off because he values communicational efficiency and doesn’t particularly care for the theatricality that INTJs often take on automatically. “You’re not a detective. You’re a drama queen!” When he sees logical problems in other peoples statements, he points them out and expects an explanation.
Inferior Ne: Since Ne is John’s inferior function, he often has a difficult time understanding Sherlock’s Ni logic jumps. However, he’s aware of this, and asks Sherlock to explain himself when he does not understand. Meanwhile, even when John doesn’t understand the little details, he can usually see the larger picture behind what Sherlock is doing. He can, in a sense, see when two things are connected, but isn’t able to connect the dots in his head the way that Sherlock can. His intuition isn’t always accurate, and when he does go on hunches, they never lead him to answers.