Answer (1 of 6): I'm a COD player, and I LOVE zombie mode. While I can only speak for myself, but I suspect many would agree with me. (And based on the user research I read, these are some of the main reasons why some people prefer PvE content over PVP). Basically, Zombie mode is a survival co-...
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The main reason I don't like competitive multiplayer game play is because it's too aggressive. People are hostile with each other, insults flying around, ego being hurt, even between team members, people are blaming each other for loses. And PvP is generally harder to get comfortable with, specially for new and lower skill level players. You're basically cannon fodder for higher level players. Matchmaking make things a bit more balanced, but in the end, it often get uncomfortable.
It's just overall a less competitive, less stressful, less intimidating game experience. It feels really good co-op with other players and survive the wave, it feels good to help someone, or revive your downed teammates, it feels good to provide assist, and even when losing, people tend to encourage each other and try again.
For PvP, most players had to work their way out of negative feelings. It is inevitable you end up being killed over and over again by more experience players. People have to spend hours of game play to get better. And the constant insult from other players doesn't make it any easier. But PvE, on the other hand, focus more on delivering a positive experience. You feel good about fighting, helping, cooperating, completing missions, winning the wave... While some might find it boring fighting against bots, but I think a lot of people would rather enjoy the game than being beat up by other players.
Translation: i'm too mentally fragile to play multiplayer