Why do people think they should win without trying their best?

This is not another CBMM/SBMM thread.

I keep seeing posts about the Crucible where people complain about "have[ing] to try [their] absolute best 100% of the time" in order to win. I just don't get this. This is in the context of matchmaking, Doubles games, getting wins in 3v3 game modes, and more.

Sure, sometimes I want to screw around in the Crucible and try new weapons or have a few beers with friends and not push myself. But I don't expect to win if I'm not trying my hardest. Especially if it's a game mode where winning makes a difference -- because then the other team will be giving it their 100%.

Isn't this how it should work? If it's a game mode where wins matter then both sides will be pushing themselves and the team that isn't giving 100% will lose. And those wins are all the sweeter becuase of the quality competition.