I'm only speaking of personal expriences from WoW. PvP wasn't always toxic in WoW in Vanilla i felt it had a mental balance of sorts on how well PvE and PvP went together at times. (So well at times that you had to be good at both if you wanted to progress or improve your character)
Then in Burning Crusade with the introduction of arenas and pure PvP gear it started to become hostile, aggressive and intense. Then in WOTLK it just exploded as Blizzard started hacking and slashing things that made classes special in PvE and PvP creating huge imbalances between classes and specs downright breaking them at times. As that was happening people blamed each other and Blizzard, as everyone just started hating each other all of the sudden. (I mean it was bad before but each time a PvP server went down for repairs they rushed a RP servers to harrass roleplayers and accuse them from god knows what. Lost few dozen friends that way.)
After that each patch and each expansion brought out new imbalances and what class or spec is the most OP and really limited the playability of the game as people started focusing on the elitist jerk method of creating a character instead of playing with the style they developed themselves as they played. (So yeah again i lost alot of people never to be seen again, not to mention how the game essentially lost it soul. If a player cannot create their own playstyle what's the point of playing anymore? New look and little stats upgrade? Personal growth was the best reward you could get with your own playstyle and when that is gone. Only greedy people remain.)
In Mist of Pandaria and Cataclysm it was so bad that there was actual whole PvP guilds that would bully and harrass people who liked anything beyond PvP or even if they tried to do anything beyond PvP. (GM's didn't do anything about it as those guilds could do damage for months on end causing people quit the game or worse yet start doing what the PvP'ers were doing before they finally banned those guys.)
Like god forbid if you had a conversation in the open with a friend talking about Blizzcon and what they spoke about fixed arena fights and how their planning to prevent that. I had a whole PvP guild chasing after me for months trying to ruin my game time as they didn't like what they said at Blizzcon and somehow it was my fault for even mentioning the fact that bunch of the PvP'ers in MoP and Cata were just fixed fighters with no actual skill in PvP as most of them were carried aswell.
7 years of that i don't want to exprience again. I don't want to see precious few friends leave every week, i don't wish to see complete utter whackjobs to be rewarded for being worst possible player they can be, i don't have the energy or the money to go trough that once again.
PvP'ers don't know how to draw a line between a friendly player or a hostile player as all players are hostile to them and they don't know how to quit PvP unless the very game is shutdown. (Personal exprience on this was how i killed this orc hunter on my night elf druid. He comes to my server and starts stalking me and trying to spam me. Then there was that PvP guild that tried to follow me around and harrass people that talked to me trying to isolate me from my friends and guild mates, which is a real dirtbag near predatory move but then again i can't really fault them from using the tricks that were forced in to their head with constant PvP where they do nothing but pile on one guy at a time then get rewarded for that behavior.)
I don't mind the aggression mind you. But when PvP is turned to something that nurtures the worst aspects of someones personality, it's like that saying about the two wolves, you grow the one you feed.
*edit: I felt i had to add that on those 7 years there wasn't a single day where i was given a chance to just sit down and play WoW without being forced in to a fight with a PvP'er. They would spam duels, insults and then try to t-bag me when i tried to sit on my usual spot while stalking me or worse yet my friends. As much as i enjoyed the 3 years of WoW that i had in Vanilla WoW and the PvP there, those 7 years convinced me otherwise that PvP will never work again as the people in PvP won't ever work with anyone beyond PvP again. (Yes total of 10 years)
Those nice days of PvP are gone.
I wouldn't mind a proper bar brawl mode.