One of my favorite cinematic "moments" of Left 4 Dead 2 occurs when some dumbass on your team decides that they're going to go scout ahead by themselves and clear the path for everyone else. They might have other reasons, but they're largely irrelevant. Because we all know what happens to the lone hero in zombie movies.
Left 4 Dead really punishes loners because if a special infected gets a hold of you, you cannot escape or fight your way out of the situation by yourself. You need a teammate to come and rescue your ass before that Hunter tears all your guts out.
On the one hand, it can be pretty gratifying to watch dumbfuck "PsychoKilla69" get eaten alive because he simply refused to listen to everyone else saying, "stick with the group", "don't wander off by yourself", "cover me", etc. Similarly, there's a sick satisfaction from watching the airheaded cheerleader bite the dust in any zombie flick. Zombies provide a uniquely rapid form of natural selection. You've got to follow some basic rules or you don't deserve to make it to that mystical "safe-house". Who wants your dumbass genes polluting the oh-so-delicate future of the human race?
On the other hand, sadly, you sort of need Dipshit. In L4D2, 3 good players can take on a zombie horde but there's significantly less room for error. Inevitably, you'll find yourself running into dangerous situations to rescue said dumbass, and five minutes later, you're wallowing in a pool of your own bile, blood, and internal organs. Where should we draw the line on self-sacrifice for our fellow human beings versus letting the Horde punish the Retarded?
Normally, I think we'd all be comfortable saying, "Fuck 'em". But there's nothing "normal" about a Zombie Apocalypse. In particular, the value of an individual non-infected human being is dramatically increased. Therefore, when calculating whether you should put your own vulnerable ass in danger to save the life of an idiot, your brain needs to take into account the rarity of humanity. This heavily skews our trusty cost-benefit analysis.
Another example of this is Battlestar Galactica. After the Cylon attack on the colonies, less than 50,000 humans survive, instantly making us an endangered species. The value of an individual human life skyrockets, which means that you have to learn to put up with idiots, pricks, and sadists. I mean, in any sane galaxy (with billions/trillions of humans), no one in their right mind would let Saul Tigh near a car much less XO a battlestar. But beggars can't be choosers.
So go ahead and jump in that sewer to save yon big-breasted bimbo. After all, you need her for breeding.