I agree. I prefer co-op mode because it's fun playing with someone else with the same goals as you do.
With multiplayer games, people tend to focus on themselves and their own personal goals instead of working with others.
The worse multiplayer experience I had was
Command & Conquer: Renegade. I would join a game and suddenly see people leaving because they didn't like the team they were on. So, I ended up on a team with half the number of players as the other team.
The other thing about multiplayer games (from what I've heard from my friends) is the obsession that some people have on achievements. For instance, one achievement involves killing everyone with each weapon. As soon as the game starts, a couple people run off looking for random weapons. As soon as they kill someone, they immediately run away to look for a different weapon leaving the serious players stuck in the game.