Recently I just go back into playing Counterstrike Online again. Its been over a decade or so since I really played it. I started playing vs bots, just re-learning the maps, controls and the differences with Counterstrike Global Ops.
The bots, even on the highest difficulty provided no real challenge. While good they just can't simulate the randomness of real people. I ran the risk of building up my perception on how good I was vs what it really was which was predicting the patterns bots have.
So I dove online in the intention to just give it a go. I died a lot but I learned a lot more and really enjoyed myself.
Funnily enough the last 10 years is about how long I have been playing MMO's. MMO's are a illusion of skill but the skill is actually all in builds, gear and rotations. Once you learn the basics in a MMO they can become trivial to play in a PvE environment. You end up just using what works the most effective, learn the routine the mobs use and just sleep your way though the game.
Very few mmo's provided any real personal challenge while leveling up. They don't want to scare away their player base and instead keep the 'challenging' content for raids and pvp. This can create a bad state of mind in a lot of MMO players who only PvE when they enter PvP.
The game has built them up for the last few months ( or weeks theses days with the length of mmo's ) to be the unstoppable hero. You slayed the dragon, pushed back the pending evil, got the epic loots only to be auto shot down by a ranger at 30 yards range in a Structured PvP Match.
This breaks your self esteem and I am sure it will cause people to just never try it again when really, you just got to go in with the perception, you are going to fail.
Like I did with counterstrike once you break this down, you end up with a much more enjoyable game. You look at why you died, learn and adapted. With some mmo's you are going to fail a lot because of the game design around loot = skill so you have to grind it out. Others like Guildwars 2 will allow you to instead change your setup and just try again.
Ultimately what I am getting at is PvE needs PvP for the simple fact as it will keep you more grounded. There will always be someone better than you and failing can be more healthy than always winning. You learn more, you get faster and you become a better player.
Lastly the main thing to remember, is its just a game, have fun what ever you do. This is just my observation on my own experiences.