That dude probably doesn't care if they were dicking around, but probably feels like trash if they were just having a bad game.

This is the most well-phrased way I've ever heard to describe why people get mad at the lack of motivation in others.

People want to have fun no matter what mode they're playing, and if they're feeling good about their capabilities, it will (usually) show in their performance. When they're not doing so well, laughing it off helps them cope with the negativity better than resorting to an obsessive desire to succeed. You simply can't win every game. Sometimes you end up on a team with five people who are ready to win, and if you're the sixth that just isn't feeling it, it feels bad for the other five, but that's just the nature of matchmaking. If you can't live with the fact that some of your teammates won't always be ready to play at the level you need to win regardless of skill rating, then you need to work on handling empathy better. They're just as human as you, and they make mistakes just like you.