I usually loathe multiplayer games, because I loathe the multiple players who I end up playing.
In multiplayer games, many players exploit game engines and use cheap strategies in a ways AIs would never do. For example, when I play CoD, I always killed by some camping douche who is safely hiding in a corner. Campers argue that “camping is a legitimate strategy because you can do the same thing”. I find that argument moot, because there isn’t anything fun about hiding in one place and shooting anyone who walks by. Of course, why play a game if you can’t have fun?
I also can’t stand the loudest, most talkative people in multiplayer games have nothing nice or relevant to say. Most of the voices and messages are a bunch of preteens who think they are cool because they know cuss words. When I don’t hear the prepubescent trash talkers, there is scummy adult or teen who will berate the 10-year-old trash talkers until they cry. Basically, the veil of online autonomy makes people unbearable to play with.
I also find most multiplayer games repetitive and kind of pointless. I like to get invested in a story and feel like I'm progressing towards some goal, but in most multiplayer games all you do is try to kill each other for a few minutes for no reason whatsoever and for no significant reward. Then once you're done, you start all over again. That can be fun in small doses, but I lose interest in it very quickly. I think multiplayer games need to learn a lesson from “Assassin’s Creed: Revelations”, and use a frame story to make the multiplayer matches seem less pointless.
While there are many aspects that I don’t like multiplayer, I do have fun playing with people I know. As a kid, I really enjoyed playing Super Smash Brothers, because the game seemed more fun when you can playfully punch the guy next to you. Also, I have more fun playing co-op multiplayer than any other game type. Co-op has the fun of a single player campaign, but you get to do it while goofing off with your buddies. The times when PvP multiplayer matches only have people you know can be a lot of fun too. Making strategies with friends and trash talking without being mean spirited is what makes multiplayer games fun, but that isn’t easy to only play with just your friends.