Because Evolution didn’t optimize us for happiness. We have been honed to survive long enough to reproduce in a variety of harsh environments, and pain is really useful to that end. It alerts us to threats. A child incapable of feeling pain would lead a short, happy life–too short to have any children of his own. Those of us who have survived are necessarily the descendants of those who could feel pain, and we’ve inherited that ability from them.

Since humans are pack animals, many threats come from within the pack. We’re obsessed with status and coupling, which is exactly what you’d predict we’d most care about, given our tribal inclinations and the fact that our children need years or nurturing in order to survive. Anything that threatens our social status or ability to mate makes us miserable, scared, and/or angry.

Being lonely hurts. That pain is an evolved adaptation. It tends to prod us towards tribal collaboration and seeking out mates. When a loved one dies, it’s devastating. Even thinking about the possibility depresses us. It spurs us to do everything we can to protect those we care about, which helps our genes and our tribe’s genes survive.

Getting a low salary or getting fired hurts, because it’s a blow to status, and low-status tribe members get last pick of the kill. It’s agonizing when we feel that many people hate us (or when we suspect they might), because for our tribal ancestors, being ostracized usually meant death. Our minds and bodies are warning us to avoid that fate.

Meanwhile, there’s no overseer saying, “None of this is fair. I’m going to make things easier on poor, hard-working folks.” The Universe doesn’t care. It also doesn’t not care. It’s indifferent. But even that’s a misleading way of thinking. When I say that Sally is indifferent, it evokes an image of a turned-up nose or a shrug.

The Universe isn’t shrugging or turning up it’s nose. It’s not busy thinking about other things. It’s not thinking about anything. It isn’t conscious. It can’t love us, hate us, or be indifferent to us. It’s simply the casing we live in and the stuff we’re made of. It has the traits that it has, some of which are threats to our genes’ survival, and our pain generally causes us to struggle against those threats.

We feel pain and struggle, because we inherited pain and struggling from our parents who inherited pain and struggling from their parents. Organisms that don’t feel pain and don’t struggle generally have no children to pass their lack-of-pain-and-struggling onto.

Perhaps the biggest cause of human unhappiness is fear of death. Most species have a survival instinct, but we have something “better” — better in terms of it urging us to survive long enough to have lots of children and take care of them until they’re old enough to survive on their own. As far as we know, cows don’t worry that they’ll get old and die. They don’t even worry that they’ll get slaughtered until it’s about to happen or they see it happening to other cows.

But our complex brains allows us to start worrying and obsessing about dying when we’re really young. A human five-year-old might lose sleep over the idea of dying when he’s 80. This is no fun at all, but it keeps us constantly planning and toiling to stave off the inevitable. Combine a survival instinct with knowledge of mortality and you get both a boon to survival (a species trying very hard not to die!) and a life of unhappiness.

We’ve also engineered all sorts of technologies that make us more miserable than we have to be. True, we’ve created tools and techniques to boost our happiness (movies, music, dancing, Disney World, Viagra…), but our peppy tech much constantly wage war against the things we’ve built that make us unhappy.

Such as agriculture. Hunter-gatherers work less than agriculturalists and they rarely fight over property, because they don’t have any to fight over. Farming is back-breaking work, and you have to settle down in order to do it. Which means you have to protect your home. Or the king has to protect it for you, by raising an army and fighting wars with invading armies. Which means there has to be a king, and he’s probably not you.

Since you now have a home and land, you want to keep it in your family after you die, so you need to make sure your kids are really yours. This means you must subjugate your wife. If other men get too close to her, a stranger’s son might wind up inheriting your farm! And you also need to make sure your wife is constantly churning out kids, because you need lots of laborers to help you plow the fields.

Many of your kids will die, because agriculture has caused people to clump together in huge numbers, allowing all sorts of germs and bacteria to flourish. We spend billions of dollars every year patching over the problems we’ve created by settling down into huge cultures: we’ve erected giant sanitation systems and must labor to keep them in working order; we need massive police forces; we need an army of garbage collectors, etc. All of these systems break, causing us misery.

Meanwhile, we destroy our health with cheap junk food that’s always at arms reach; we’ve created an economic system which is rigged to give some folks immense advantages while leaving others to scrounge on the streets; we’ve created weapons so powerful, they are capable of destroying our species; and we’ve decimated the environment.

Life is rough because planet Earth was a harsh place before we arrived and because we’ve toiled to make it harsher.