Non-Coercion And Conflict
Anger is a powerful motivator. From our evolutionary history, Anger likely evolved to protect us from predators and to motivate fierce competition over discovered resoruces. The emotion of anger is triggered when our plans are frustrated or when our expectations are violated. In response, we locate some nearby human-like aggressor, or hallucinate one if none can be found, and attack them in hopes that harming them will allow our plans to proceed unhindered.
In a social environment, anger can easily escalate. Once one person identifies another as an aggressor, rightly or not, their attacks on the perceived aggressor mark them in turn as an aggressor to their aggressor, fueling mutual attacks until one side submits or is destroyed.
A society organized around non-coercion critically thwarts anger escalation. Non-coercion requires that all people have the real option of withdrawing from any interaction; conversly, all interactions require the consent of all parties. If all interactions are voluntary, then if ever another person frustrates your plans, you always have the opportunity to withdraw and execute your plans elsewhere. If you persist in an interaction, then you are at least as much at fault as any aggressor who you choose to interact with. To more cleanly cover all triggers to anger, non-coercion also requires that all promises be kept, and marks any breach of promise or fraud as tantamount to coerion.
If a society adapts the non-coercion principle, if its justice system and its individuals adopt expectations centered around consent, then coercion becomes the sole source of all anger, and injustice is identified precisely with coercion. The human urge of punitive justice, rooted in anger, aligns towards ensuring consent, which it can reasonably achieve. Any act of coercion inherently must have some person who coerces; a justice system based on consent punishes coercers and grants freedom wherever it falls.
Social Expectations
Imagine a society which expected something besides consent; imagine a society which expects that, every day, whenever two people meet for the first time, they must smiile at eachother. At glance this seems like a kind rule, a harmless expectation. But simply granting the expectation justifies anger if that expectation is breached. If someone is distracted and they forget to smile, or if they are having a bad day and cannot find it in themselves to smile, or if they are an honest person who feels morally compelled to not fake their feelings, any of these situations may be met with socially justified anger.
Social expectations are not free. Each social expectation must be paid for in anger, in conflict. The expectation of consent inherently reduces conflict, so that it pays for itself. All other expectations, no matter how utopic they sound, cause more conflict and inhibit peace.