Sure, you've played Left 4 Dead late into the night, with friends or other online players, or maybe even just you and a friend trying like hell to finish "No Mercy" with the dumbass computer controlling the other two survivors, but have you ever taken the time to notice all the life lessons Left 4 Dead has to offer?
1. When Life Hands You Lemons, Make a Pipe Bomb!

Or at least a Molotov! Anything! Heal yourself with it! Do something? How the hell did you find a lemon anyway? Where was it? Are there any more? Look, whatever you fucking do, DON'T WASTE IT!
2. When You Come Across a Sobbing, Psychotic Sociopath, Do Your Best to Avoid Them!

When you're out walking and see someone sitting alone and having a good, public cry, let them! Be smart, sneak around them, go the long way, whatever you do, don't walk up and say hello, or they will tear you to pieces with their long lady fingers.
3. Don't Get Puked On!

This seems a little obvious, but once someone throws up on you, you look like a douche and everyone comes running to check it out. And don't go blaming the fat guy, you already look stupid and smell like the inside of a homeless guy's cardboard shanty. Don't make it worse.
4. Louis is a Fuck Up.

Everyone has a Louis in our lives. That one friend who constantly needs help, who's like a magnet for trouble. Whether he's setting off car alarms or constantly getting pounced on or deliberately running into fire, Louis just doesn't have his shit together. You try to help him, but he'll never learn. He's going to wear down your resources, but that's okay. Friends are important.
5. Be Sure You Need Help Before Calling For It.

Whoah, whoah, hold up there, Mr. Hasty! Have you done all you can do on your own? Have you made a good plan for when the shit hits the fan? Because help takes their sweet time. Help doesn't arrive when you'd like them to, but when it goddamn feels like it. So make sure you've done all you can before you start crying for help.
6. Downing a bottle of pills will only make you feel invincible for a short while.

So you're "not doing well" you keep telling everyone. But don't fall to temptation! Your friends are going to put pain pills in your hand, but it's up to YOU whether you take them or not. Peer pressure is a bitch, and sometimes your only friends in the world will encourage addiction. Don't listen to them!
7. Sometimes It's Okay to Interrupt What Looks Like Humping.

At worst, you may be a little embarassed by interrupting an actual hump. At best, you're doing someone a favor. Trust your instincts. Sometimes what looks like a good touch is actually a really bad one.
8. Don't Light Your Friends on Fire!

OK, it's funny the first few times. Hell, the first twelve times, but jokes get old. Suddenly, you're the only one laughing. And that's worse than getting puked on.
Now, this lesson is two-fold. Clearly, the first part is to not set your friends physically on fire, the other lesson is not to light the area AROUND them on fire. Let's try communicating people.
9. When You Host a BBQ, Close The Back Gate!

Seriously. Unless you're prepared to feed the whole neighborhood!
10. When a Plane/Boat/Helicopter/Vehicle Arrives to Save You, Run Like Hell or You Will Be Left Behind

Let's face it, if you need a helicopter to come save you, you're in serious trouble. This is no time to be a hero. Hopefully you took your time before you called for it (see #5), but if you don't get to the help in time, sorry Charlie, or whatever your name was, no one is going to double back for you. They've helped you long enough. Friendship and cooperation end once help has arrived, and if you miss the boat (literally), well, have fun on your own, amigo.