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My first mmos played (UO and L2) had open PVP and it was regarded as essential part of the enviroment, indispensable to the social, political and economic dynamics of the virtual world. People in almost totality did all the other things provided by the server (pve, craft, etc) preparing to pvp in search of resources, domains, ownerships of castles/fortress/territorys, glory, respect, rivalry, revenge, fun and etc.
I only made contact with people that hate PVP after joining WoW and reading foruns. This was a novelty to me and let me very intrigued and curious about the reasons of the people that avoid PVP. Reading opinions and various texts in the internet, i saw that this population "pvp-hater" can be subdivided in 2 categories: people that likes or is indiferent to pvp itself, but hates the "free pvp" ("open world FFA pvp") and people that are truly scared of any type of pvp, even the structured. I came to a conclusion about the ultimate (or primary) causes that make PVP be disliked by some people:
1- PVE characters stays in certain spots waiting to be approached, while players can come to you.
Keeping all the other factors equal, is this the main "annoyance" of the pvp to much people. If the monsters stayed wandering the world, invading cities, aproaching players while they are occupied, distracted, no prepared or no "in mood" to fight, this "annoyance" would be regarded as a normal thing and pvp would be less loathed by these people. These monster and other AI NPC attacks would be considered "legitimates" and source of thrill, adrenaline and adventure, challenging the players in strive to survive, dodging or fighting the threats. But the same, if made by human-controled characters, is (consciously or no) regarded as "illegitimate", "desnecessary", "harassment", etc
But in 99,9% of today mmos, the PVE characters stay stopped in specific spots waiting to be slaughtered like sheeps in the pasture (see item 2).
This item explains the complains about "balance", zerg and trans-level kill too: PVE characters can too be "overpowered", in greater number or higher lv, but they stay in their spots, allowing the players approach them only if in advantageous conditions.
2- Human-controled characters is much more harder to defeat in 99% of the time.
As the AI tecnology is still far away from the capacity of a actual human brain, the human-controled characters, in equal conditions, will be more harder to defeat and unpredictable. To defeat a human-controled player requires a minimum of training, attention and motor coordination from the players, ie, skill and mental work, which not everyone is willling to do or have. Some people (i dont know if the majority or no) wanna play a mmo to just "relax" and do unchallenging things as pastime, like knitting.
If the AI controlled characters had the same complex fighting behavior and tactics of the humans, the pvp would be less hated by these people too, since there wouldn't much difference.
PVE characters can only become "hard" with gigantic increase in their stats, but still are predictable like a bull in the arena and can be easily beaten by a group of players whose only "challenge" is to form and keep a group able to memorize a handful of "movements".
3- In most mmos, PVE offfers more guaranteed rewards.
This is the main reason to much people. In 99% of the mmos, the PVE offers game rewards in a regular and guaranteed way, while PVP dont, or, if it offers, is at a high risk that sometimes dont compensates. Many mmos fails in create a functional risk-reward system to the pvp, and ends up not attracting the population that dont pvp for fun, but could happily (?) pvp if the potential reward is tempting. These mmos with bad risk-reward systems makes the pvp be practiced only by the people that have fun in doing it regardless of the "reward".
The sandboxes mmos almost always creates a good risk-reward system, since the players competes by countless things in the open world.
In summary: pvp requires more work, attention and skill to be handled (either fighting, fleeing, dodging or avoiding) and not always gives valuable rewards to tempt the people turned away by these requirements.