We humans like to see ourselves as unique among the world. We are the only animals to invent things as distinct as opera and pneumatic hammers. We come up with things like candy, religion, and the Internet, and it makes us feel very special and different.
But if you ask a neurologist, you'll see leftovers from previous brain developments. We like to think of ourselves as a solidly different mind, but the civilized human is just the top layer. It's bolted onto a social-climbing, tribal, fang-baring, poop-flinging, chimp. And the chimp is bolted in turn on a territorial, furious, horny lizard. And the lower levels often show through routinely.
If you've ever shaken a fist in fury because someone cut you off in traffic, that's the lizard. It sees the space as yours and resents this other jackass's intrusion. It's wired to pick a fight with your competitor to force him or her to leave your space. The reasons vary by your gender (a female lizard wants a safe place to raise her eggs, a male lizard wants to show how well he can defend his mate's nest), but it will do little good in a car. After all, you're both in a big steel cage, moving at high speed. Attempting to fight will at best only injure yourself pointlessly. (That and both of you will have literally vacated the spot in 30 seconds if you just wait anyway.)
If you've ever had the urge to have a big party on Wednesday night despite having work on Thursday, you have the lizard to thank for that piece of impulsiveness too. Lizards don't really benefit from planning ahead, so they want to get all their impulses done NOW. And they aren't complex impulses. Stuff your face with food, find a mate and make some more lizards, and maybe modify your mind a bit with some fermented fruit, or that interesting cactus. That we humans have many more ways to modify our mind, usually to our detriment, doesn't help matters.
If you've ever wanted to throw up because you have to give a speech (be it for school, or a business presentation), hey, it's the lizard yet again. It's saying "Hey, don't do this status grab, fool, you might get killed." That the social consequences will be worse for not doing it doesn't occur to the lizard. After all, lizard social options are kind of limited. (Mostly to "bite that asswipe," "Ignore him," "Hey, check out my bod," and "mine, MINE MINE, back off or I'll kick your ass")
If you've ever blown your paycheck, it's the lizard yet again. Lizard doesn't grasp why you shouldn't just leap into the trough and eat until you can't move, or whatever it is that you want to do and it only kind of understands. The idea of "saving" doesn't really make sense to it. And "planning for the future?" Ridiculous, if you don't go get yourself a huge stash of bugs in your belly you clearly won't survive the inevitable famine. What do you mean food costs "money?" Clearly you should trade this "money" for more food, else what good is it, right human? What do you mean this "i-pod" thing cannot be eaten? If you can't eat it, why else would you want it so badly?
If you ever leered at a person until they got pissed off, ruining any chase you had with them, guess who's back? The lizard wants to make more lizards. Lots more lizards. Too much is not enough. This gives men the urge to check out the rack of every woman he can see, just to make sure if she'd make good kids with him. Women might be more discrete about this, but I've heard many a rant on a guy having a "spankable ass" and "awesome muscles." Somebody's leering.
If you've ever put off something until it was much too late and then had to stay up a week to do it, hello again! This doesn't feel good, why are we doing it again? See, to the lizard, everything that feels good directly influences their survival. Shoving everything edible into your face? Awesome, you survive the famine. Making more lizards? Awesome, once the eggs are laid they take care of themselves. Getting totally blitzed? Well, you didn't have anything to do until this evening anyway, and this will make the daily sunning much more fun. Consequences? Consequences are for mammals, who have to RAISE their offspring, whose veins will clog if they get too obese, and so on. And that term paper? Lizard cannot even grasp such a concept.
Now, these might sound like the lizard part is a bad thing that should be eliminated as soon as possible, but we didn't keep it for no reason. If you have a gibbering fear of death, that's the lizard. Being dead sucks, but if you weren't afraid of it, you'd step in front of a bus the moment things got hard because corpses have no problems. Same for the urge to reproduce. It's a lot harder for us humans, who don't just lay eggs and leave like they do. Without a little baby-craze, humans would be a rather uncommon and embattled species.
Now the monkey is a tad more complex. It understands the idea of having relationships with others of your own kind besides picking fights and mating. It grasps that cooperation is the best way to survive, so it encourages you to form a tribe. The tribe will help each other find food, develop a guard shift so that everyone can sleep but there is never a time when everyone is asleep, and smooth over some of the fighting. It gives you the more complicated emotions, like happiness, shame, and affection. So far, so good.
Unfortunately, the monkey often has strange ideas about the purity of the tribe. Any outsiders show up? Scream and hurl poop until they run away. In us humans, the poop-flining is now metaphorical, thankfully with invective and rude hand gestures. (Man, wouldn't it be horrifying if you angered somebody and their first response was to take off their pants and poop into their hands?) Anyone contradict you? Scream and hurl some more poop, that'll teach them. Someone too different from you? Kill, kill, before he affects the others! Rrarrrgh!
You see, the monkey started with the correct idea of mutual defense, but it's incredibly stupid and impulsive. In nature one must make snap decisions, but in any decent society, these decisions are the very avatar of wrong. Monkeys have no police, we do.
The monkey is also responsible for moronic dare taking. From an objective viewpoint, almost all peer pressure revolves around unbelievably stupid crap. If a complete stranger dared you to, let's say, cram a broken bottle containing a firework into your anus and then light it, you'd rightly tell him to go fuck off. But if your best friend dares you, hurrr, okay! This is because the monkey is all about status, and will do any moronic thing to keep it. Oh man, if I don't drive this car off a cliff into the jagged rocks below, my friends won't respect me anymore. I can't be a wuss in their eyes, I'll do it!
Now, all of this is not to condemn humankind to eternal stupidity. Since the lizard and the monkey are seperate, you can play them off each other for best results. If someone pulls a gun on you, the monkey will scream "fight! Fight!" whereas the lizard will tell you "run, moron!" Go with the lizard. If you have a term paper due next month, the lizard will be like "Meh, this isn't fun," but the monkey will insist "Do it, it's important!" Go with the monkey.