One of the things I've noticed about John is that he seems to need someone else to make his life worth living. He says it about Mary, "She completely turned my life around", and his life goes to hell without Sherlock-- when we first meet him, he's contemplating suicide, because he's been invalided home from Afghanistan. 

So, Sherlock and Mary assert that "he chose" mary because he's abnormally addicted to dangerous people and situation", and I'm going to add that he sufferes debilitating depression when he doesn't get his "fix". 

Co-Dependent to say the least. 

Any diagnoses?

Davina wrote:

PTSD combined with being somewhat institutionalised from being in the forces.

So that would account for the co-dependence? And the inner pull towards dangerous situations and people?