That's what we call a vicious cycle - incidentally, a logical fallacy as well. It's similar to how tribesmen who are strangers will meet each other. At first, you are cautious, but this is another human. Your first reaction may be to communicate, to trade, to share. But all you need is for a few such meetings to lead to violence and then everyone must be quite wary in new meetings. Will this encounter result in combat? Should I surprise my enemy and attack without provocation to ensure my survival? More encounters lead to combat, and everyone knows it, so they view the danger of combat as more likely, and thus their initial reactions become less friendly as a defense. Soon the assumption on all sides is that an encounter with a stranger will lead to conflict at least, combat at worst.
To reverse the cycle you need to have people who approach all encounters in a friendly way and do not attack unless attacked. But these people are more likely to be attacked by their non-diplomatic counterparts. So they generally die out before establishing a virtuous cycle with their neighbors. And when members of the diplomatic tribe see their fellows dying, they may become angry and seek revenge, but will at least view the diplomatic approach to an encounter as less successful and more dangerous than the combat approach to an encounter.
One way it might work is if a much stronger tribe interacted with its neighbors. When six or twelve diplomatic men approach two or four hostiles, the hostiles will flee, attack, or parley. If they attack, they will generally lose and their tribe will begin to recognize combat as an ineffective strategy - at least when dealing with these people. Fleeing is a neutral result, with no diplomatic impact. Parleying means the normally-aggressive tribe sees a positive outcome and benefits to not attacking, meaning they may be more likely to consider parleying even when dealing with equals or inferiors - after all, you may have more to gain from someone than whatever they carry and neutralizing the non-threat their existence poses.
However this would be pointless as long as tribes are still shitty to each other, parleying only when encountering the vastly superior tribe. Over time would their interactions slowly become less instantly hostile? Or would they continue to attack on sight, believing that the other weak tribe will also attack them on sight? And would any such vastly superior tribe actually operate that way in reality? Or would it subjugate and slay the smaller tribes in the normal course of responding to their stupid violence with much more powerful violence?
Which is to say, if every caveman not in your family that you meet tries to beat you up, how long before you just assume all cavemen will try to beat you up unless they're part of your family?
There's an additional layer, of reputation. You could argue that reputation can be a noble thing, but that's a pretense. Your reputation in this sense is whether it is known that you can be attacked with impunity. Take a gangster who gets insulted. Will he accept the insult, letting his peers know that they can insult him at will and he will do nothing? In that way he becomes known as weak and loses social standing, inviting attack. This is why thugs are so quick to react with violence to any perceived lack of respect.
In this case, let us say your tribe is not especially strong, but has a very solid diplomatic background. Your tribe will always try to parley and negotiate instead of ambush and pillage. A neighboring tribe occasionally finds itself near your hunting grounds and takes many animals from your traps. One day your hunting party meets their hunting party, and they hurl stones and chase you off. You try to shout and approach to talk it out, to tell them you set those traps and you expected to take those animals. They respond with more stones. They generally choose to fight with you instead of negotiating because (1) they don't care about you because you're not their tribe, (2) you have resources that they want, and (3) they believe you may be aggressive toward them and are wary of tricks.
Let's say you convince them that you will not attack, after suffering many casualties. What does the other tribe think? (1) and (2) are still in full force, but they've reversed (3) and now understand they can do anything to you and you won't fight back. They slay your diminished hunting party, enter your village, slay your remaining men, and seize your women and goods. They gain a reputation in the valley as having decimated another tribe and taken their hunting grounds, meaning more of their enemies will choose to flee rather than fight. They're also much larger and with the greater population and food sources will quickly grow much stronger. So it is that any tribe would have a great incentive to pretend to be friendly and then ambush their rivals. And the other tribes know that diplomacy leads to ruin. All of which make it even less likely that anyone else will attempt diplomacy.
Only when combat and conquering means very little gain and much loss of life and material, will tribes decline the naturally-effective model of warfare in favor of diplomacy, and even then may shift to war if they don't get what they want.
This extends to interpersonal encounters in civilized society. If you see a person who walks out of the bank with a hefty bank envelope, you would stand to gain a lot by robbing him. Looking him over, it's clear he's not armed, and if you are, that's an enormous advantage. Surprise is just as much, if not more, of an advantage. But there is a portion of your tribe set aside to prevent crime and punish criminals, reducing anarchic behavior, and making society more civilized and comfortable. In fact, one could argue that it is only because of that police power that such things as an unarmed citizen, a bank with money, and someone walking to their car without taking precautions against being robbed can exist. In a state of anarchy conditions quickly revert to a violent and dangerous "all against all" in which there is much waste and tragedy.