I’ve done a bit of fighting in my day. Brawling with siblings and friends is akin to mixing it up with your comrade in arms – i.e. shipmates, barracks roommate, etc. You might be trying to bust their nose or blacken their eyes but for the most part you really don’t have the intention to inflict permanent damage because subconsciously you know the person you are fighting and up until that point of violent conflict were at the very least neutral to each other or on the same side which essentially inhibits the full effectiveness of you tenaciously fighting them. The same condition applies when training (regardless of how intense the training is) in any martial art whether with or without weapons. – Control to a certain measure is applied often times in various degrees within each combatant’s psyche in addition to outside influences – authorities showing up to end the conflict, referees to limit the engagement (tournaments, boxing/MMA matches, etc).
However in an actual fight (bar fight, street fight, and combat?) there is the fear factor that either overrides those ‘safety’ governors or induces a flight reaction. It is the element of the unknown concerning your adversary’s intentions, skills and tenacity that invokes the fear within, as it should for all you know you are about to engage in battle with your killer. – A sobering thought that demands you give it careful consideration of what extent you will carry the fight. The example you provided Nate is a good start. You stated that it opened your eyes. How so? A headlock is a basic ‘unskilled’ attempt to control and subdue an opponent which relies upon exerting leverage upon the head using superior strength. – If it opened your eyes then you should realize that you need to learn how to quickly and easily breakout of these kind of basic holds or convince your opponent it is in their best interest to immediately release you.