In Buddhist terms we could say that we suffer because we are selfish, especially in relationship to sensory stimuli. I've explored this in a number of blog posts recently.  In order to find happiness we seek to obtain, maintain and retain pleasurable experiences. These are, however, inherently impermanent and unsatisfactory so that we find life itself unsatisfactory. But why are we this way? Why evolve a faculty that only makes us miserable?
Actually as social animals, despite our sense of being independent selves, we are not inherently selfish: rather we are instinctively gregarious, cooperative and empathetic. As humans, indeed as primates, these are very much part of our genetic heritage. Although there is conflict and competition in all primate groups, they are characterised by a high level of helping each other and working together for the benefit of the troop. So why do we become selfish? I think that the problem is a result of our own success - or because our success at exploiting the environment has outstripped our genetic evolution. We are genetically adapted, to take two examples, to scarce resources (e.g. diets low in sugar and fat) and small group sizes. Pleasurable sensations help motivate us to find and assess the goodness of food, and to contribute to the social group through, for example, cooperation and social grooming; while unpleasant sensations helps us avoid spoiled food and danger for instance. In short we are programmed to experience pleasure as happiness because in the world that we are genetically adapted to this makes us more successful.
About 10,000 years ago we humans began to use our ability to think ahead to our advantage. We began to cultivate food crops rather than scavenging, and to domesticate animals which we had previously only hunted. The result was a reliable food surplus for the first time in history. It was still somewhat related to climate patterns - drought was not unknown - but we could mitigate that through irrigation. We ate well and as a result grew stronger, lived longer, and our groups began to get larger. We began to make large scale permanent dwellings - the first cities seem to date from around 9,000 years ago. Large scale cities with hundreds of thousands of residents became possible as agriculture intensified. Civilisation provides many benefits to us individually and collectively. Importantly it makes reproductive success more likely, much more likely, which is positive in evolutionary terms.
It is sometimes said that humans have stopped evolving but this is not true.  It is true however that our cultural and technological evolution has outstripped our genetic evolution by orders of magnitude. In most cases we live in an environment to which are not genetically adapted. This is the result of a trend that began thousands of generations ago, and means that we have to consciously adapt to our circumstances using our ability to learn and innovate. As societies become more complex, we have to be better at learning and teaching these acquired skills because our genetic adaptation is less relevant. It's a self-reinforcing cycle, and the speed of change is increasing!
In a world of generalised surplus the relationship between pleasure and happiness becomes more abstract.  Once the relationship becomes abstract then it is a bit like abandoning the gold standard behind money - it's difficult to know the value of anything. The result is that pleasure becomes an end in itself. Similarly any pain, or the lack of pleasure, is bad and to be avoided. This gives rise to two extremes: on the one hand we theorise about an absolutely abstract ultimate pleasure (or equally an absence of pain) which awaits us (usually) in an afterlife; on the other hand we might decide or there is no greater good than pleasure here and now. These are the two extremes of eternalism and nihilism.
As group sizes soar we not only split into increasingly disparate factions, but we become accustomed to being surrounded by strangers to whom we have no social ties - they are not related and not part of our troop and we owe them nothing. Larger social groups require new social structures with arbitrary relationships. We may never meet those who lead our community for instance, or even their deputies. I've never personally spoken to a member of parliament of any country for instance. The result is alienation and a feeling of disconnection between us and the people around us.
So we find ourselves pursuing pleasures with considerable energy and ingenuity, but surrounded and led by strangers, and over several hundred generations this becomes the cultural norm. This is our norm. It creates a deep dissonance within us - emotional as well as cognitive - because we are overstimulated on the one hand, and alienated on the other. We find ourselves plagued by diseases caused by diet such as heart disease, obesity, bowel cancer and diabetes; by drug problems, alienation and depression; and by conflict, crime, civil strife and violence. To some extent this is balanced out, though, because at the same time this dissonance has driven the production of great art, music, literature and drama as people try to give expression to something more wholesome. However we are left with a considerable and worsening problem.
Eventually some individuals began to emerge who used their powers of reflection to examine the human situation. During the so-called Axial Age (ca 800 BCE - 200 BCE) many such individuals appeared including Lao-tzu, K'ung-tzu, Isaiah, Zoroaster, Yajñavalkya, Mahāvīra, Gautama, Pythagoras, and Socrates. One thing they all seem to have done is call into question the pursuit of pleasure as an end in itself, and encourage us to relate to each other in more wholesome ways. The greatest of these individuals was Gautama, the Buddha - he saw the nature of the problem more clearly than any other human being before or since. Since the Axial age we Westerners have swung between puritanism and hedonism, from eternalism to nihilism in response to our inner dissonance without any great success in quelling it. For some time now some of us have been exploring the Buddha's middle-way, although in Britain's last census more people identified their religion as Jedi (0.7%) than as Buddhist (0.3%).
Neither hedonism nor puritanism address the underlying relationship we have with sensory stimulus, especially pleasure, so neither can resolve the fundamental dissonance, nor produce lasting happiness. The extent of suffering in the world (various 20th century genocides for instance) makes belief in God untenable for any thinking person, but the abandonment of old values in reaction to the loss of faith has had a devastating effect on society. Plurality has lead to moral relativity and reinforced the confusion over values. The sad truth is that as much as some of us find the choice and variety of contemporary life exciting and stimulating, the majority feel overwhelmed and anxious or angry (fuelled in part by a media with a vested interest in stimulating precisely these emotions). Increasingly people are closing their minds and hearts - or turning for example to drugs ; or the ersatz, but less challenging, community provided by the internet. 
So we suffer because, as a side effect of civilisation, we have an aberrant relationship with sensory stimulation. Instead of experiencing ourselves as being part of a complex web of relationships with people and the environment, we feel isolated and alienated. We are overstimulated most of the time, and continually stoke the fire because we are convinced that pleasure is happiness in a generalised abstract sense. Selfishness is a by-product of this process, not a cause - which is to turn traditional Buddhist narratives on their head. Civilisation has been a two edged sword which may suggest why periods of barbarism punctuate the history of civilisation. Buddhist practice offers the best way forward because it directly addresses these problems with practical methods and suggestions. 
- Examples of recent posts on our relationship to the senses include:
- see for example 'Humans are still evolving - and it's happening faster than ever'. The Guardian 11.12.2007.
- Here I have to make a broad generalisation which glosses over some important questions such as endemic poverty and whether the subsistence farmer is better off than the hunter gatherer etc. Certainly agriculture is at different stages around the world (I've seen farmers using all-wood ox-drawn ploughs in India for instance), but there has been a general trend towards more sophistication. My remarks are intended to apply mainly to my audience who I take to be English speaking internet users.
- It is ironic the extent to which terrorism, supposedly the greatest threat to our society, is funded by western drug habits - certainly Middle-Eastern terrorists are funded by opiate production, and opiate production is driven by the demand for illicit opiates in the west.
- See my comments on virtual community [19.9.08]
- Although Buddhist practice is the overall theme of this blog I did summarise the entire Buddhist path in a way which is relevant to the current post in another two-parter back in 2005: - part one (generosity, ethics, and patience), and part two (vigour, meditation, wisdom).