Lost all your lives while playing Contra? Just steal some of mine. That goes for any Mario game, too. Got your ass handed to you by that last horde in Left 4 Dead? Here, use my first aid kit. Yeah, I know my health is just as low as yours, but just take it. What's that? Now you're being strangled by one of those stealth ninja douchebags in Uncharted 2? Worry not, I just bear tackled him and broke his neck. You're welcome.
Call me selfless. Call me a martyr. Call me a team player. Call me Jesus. Call me whatever you like. I'll be the one saving your ass.
I may not have the highest kill count at the end of an Uncharted 2 co-op match, but I will always be the one keeping my teammates alive and in the action. One particular survival match on the Village level comes to mind in which my teammates consisted of an experienced Level 40 player and a Level 1 noob. The L40 acted like he was playing single player, focusing solely on racking up kills and generally ignoring the two of us. This left me to babysit the noob, and babysit I did. The poor kid was running around on ground level getting consistently dropped by the flooding enemies. This meant I had to continually endanger myself to bring him back up. After a while though, I was having to snipe those strangly ninja fucks off of both of them in addition to reviving the noob every so often.
It's definitely a thankless job. I've noticed especially in U2 that some people never seem to extend the same courtesy.
Hyperbolic image intended to better illustrate my point
Another time during a 5 player Killing Floor match, one of my squadmates was attempting to reach the safety of the bottleneck we had created when he fell from our 2nd story base into a pile of specimens lingering on the first floor. I didn't think twice. I jumped down into the fray with him (losing around half my health from the fall alone) and opened fire. They were everywhere, and back-to-back we emptied our magazines into the gnashing wall of flesh until we were both killed.
I will rescue you or I will die trying.
While that may sound noble, it's actually quite neurotic and borderline stupid behavior. Again, with Uncharted 2 or Left 4 Dead, I'll always save people even if they don't return the favor, or don't share their first aid kits, or if they continually act stupid and repeatedly get killed. Sure, I'll be pissed about it, but I'll still do it. This is more masochism than nobility.
While I'd like to claim that it's just my good-samaritan nature, I'm not actually sure why I feel compelled to always be the one rescuing people. Maybe I'm like an autistic kid and it's the control that I desire. Or it could be that annoying idiosyncrasy where I need to make sure everyone is having fun or I won't have fun. Or perhaps it's just an attempt to make up for my lack of skill in other areas. I'm an above average marksman in any given FPS, but can't compete with the twitch masters.
Maybe If I can keep my teammates alive and functional, and we still lose, then I can say that at least I did everything I could.
Regardless of my reasons, saving other players is a metagame at which I excel. So if you want to stay alive, I'd best be on your team.