There have been several features dedicated to the psychology behind fighting, the fact that humans are actually hardwired to fight and survive. The fact is, it's not only psychologically inherent by nature, fighting is also physically inherent by nature.
Recent studies published by the Research Quarterly for Exercise and Sport, has shown that contact sports can actually cause your body to convert cells into lean tissue. The demands of high-impact sports, like boxing causes the brain to send signals to the body that it needs to be protected by muscle instead of by fat. As a result, this conversion occurs at a cellular level.
This, in theory, isn't all that different from bodybuilding or muscle strengthening in general. By placing a high demand of weight or volume load on your muscles, they harden, grow and become stronger. The breaking-down or tearing of muscle tissue, forces the muscles to rebuild stronger and bigger.
This finding has very specific applications to boxing because it is about as high contact and high impact as you get in sports. The message here is that your body will adapt and conform to do as much as you ask it to do. Part of your job as a combatant is to create a suit of armor you can take into battle; one that will not only inflict damage, but will also help protect you.
There are several approaches you can take in the gym that will aid in this toughening process. One method is Body Sparring.
You obviously can't spar and shouldn't spar all out, all of the time, but you can set some ground rules to simulate the rough stuff to some degree.
Going full power, full contact, and all body allows you to throw hard and take power shots, without all of the unwanted side effects that come with repeatedly getting hit in the face or “rocked”. Gym wars prepare you well for a fight, but they have long-term, negative repercussions when they happen too often. Taking head shots out of the equation, allows you to toughen your body and condition yourself to get a little rough. If, either you or your sparring partner, can't take the body shots, then that's a sign that its work you should be doing more of anyway. If you get "stopped" by a body shot, better to have that weak link exposed in the gym, rather than in the ring. Arms, shoulders, abdomen, upper torso (head excluded) are all fair game in this exercise.
Aside from this specific drill, don't pamper your body during training. Don't do things that could lead to injury, but demand that your meat suit do hard things. Make it endure some contact, some banging around and a little bit of pain. That’s the only way it will come to understand what you expect of it.
Keep in mind, the human skeletal structure has 640 muscles wrapped around your bones. Your goal in 80% of your work outs should be to stress and strengthen as many of them as possible. The more physical you get, the deeper you'll be tapping into your ancestral roots. You'll be building-on an inherent ability to toughen-up to take whatever comes your way.