I always think of Plato's Republic when reading Reacher.
I'm no scholar and it's been a long time so feel free to correct me. As I recall, Plato describes an ideal society. It would be ruled by highly educated people well versed in philosophy. The general populace he labeled Artisans. A third class was the Guardians. These would be the soldiers, the warriors.
Unlike the Artisans, the Guardians would have little interest in material possessions. They would value strength, skill, honor, justice and courage. He didn't want an army of blood thirsty savages that might turn on the citizens so he describes the perfect Guardian as one who is aggressive but also possesses the opposite quality of gentleness. In addition to physical training, they should learn the value of literature and music.
Reacher is a Guardian. He was born to a Marine and lived on military bases all over the world as a child. He learned to harness his natural aggression as a way to cope with the problems of being the new kid at school. He joined the army and received extensive training as a warrior. All he'd ever known was the soldiers life. He was content to serve his country as an officer in the Army MPs.
Post Vietnam, the military downsized. For the first time, nobody came around to ask Reacher to re-enlist. He wasn't accustomed to making choices about his future. The easiest choice was to do nothing. This resulted in the end of his military career. With little interest in possessions, he'd saved enough money that he figured he could just wander around the country awhile. Most of his life had been spent overseas and he wanted to see the country he had been protecting.
So here you have this classic Guardian wandering the country side, walking, hitching or riding the bus, who owns the clothes on his back and a folding toothbrush. He no longer has a chain of command to point him at this battle or that war. He's uncomfortable choosing for himself so he lets chance guide him. He's not looking for a fight but he's not about to run from one either. He's intelligent with years of investigative experience. I enjoy watching him work. I think Plato would also.