I would quote alot of people here but there's seriously too many.
The main problem with FPS's is the fact that because it's a game you get a feeling of invulnerability.
I mean there's no way in real life a commander would be airstrike the exact same spot his/her troops were fighting on.
There's no way in real life you'd run round a corner with a grenade in your hand if you weren't insane.
There's a very low chance that a grenade would magically unpin itself the moment you die.
You wouldn't know the ins and outs of the battlefield you're fighting on because chances are you've never been there before.
There's NO WAY anyone's ever going to invent a gun with a CHAINSAW as a bayonet that any army will actually USE.
If all else fails, so what you died, wait a few seconds and RESPAWN. This is the problem, because you don't actually DIE it's never a threat to yourself.
Also most FPS's are unrealistic. In most real-life combat situations there would be ALOT more inter-team cooperation and everyone would be constantly talking. Where as online you get one guy who can wipe the entire team because he sat in the right window at the right time.