Why are we here? Is there some great plan which we are here to fulfil? Is there some ideal set of values we should espouse? Or are we a multitude of Sisyphi, pushing boulders uphill, only to watch them roll back down. “Why are we here?” is the question that typifies philosophy - it's certainly one that you tend to get asked when people learn that you study philosophy. The irony is that the question is essentially settled. It's a dead end issue, just like Newtonian psychics.
By settled I don't mean that we can say that this or that is the answer, rather that the last major innovation - a new answer to the question as it were - was laid out about 200 years ago. Since then it's been a philosophical sitzkrieg. This has not made it a less interesting question though! In this essay I'll touch on all of the families of solutions, but I'll spend more time on nihilism, simply because I think it is both the most misunderstood and the most right. It also happens to be the creed that I subscribe to.
Essentially, the questions being asked - very distinct questions which usually have pretty much the same answer - are “Why am I here?” and “Why are we here?”. There are three classical answers.
The first, and oldest, of these is divine purpose, aka “God's Plan”. Divine purpose is the hypothesis that god created us (collectively and individually) to fulfil some plan. Each and every one of us is a small cog or jigsaw piece, without which the plan would never be completed. The first, and most obvious criticism, is that there isn't a god. However atheism is not exactly a universally accepted viewpoint. So let's assume for the moment that there is a god. Maybe even a God.
Next, an outline of the argument from evil, and the traditional response to this argument, before I tie everything together. Essentially the argument from evil points out the apparent contradiction between the idea of all-loving, all-knowing, all-powerful gods and the existence of evil (disease, famine, little children getting cancer etc). The conclusion from this is that the world we see around us and the existence of such a god contradict, and that it is god that does not exist, rather than reality. Your views may differ.
The traditional counter argument to this conclusion is that the evil is necessary in some way to fulfil god's plan, or that it only appears to be evil, when in fact it is not. In essence, the response is some variation on the general theme “this is not something we can understand, it is merely something we have to accept”. Ignoring the flaws in this counter-argument itself, what happens if you give up knowledge of the divine plan in this way (the god “moves in mysterious ways”)? You run into a fairly serious issue: In making the divine plan unknowable, you have essentially taken away it's ability to explain your life. We cannot answer the question “Why are we here?” with an unknowable plan. We cannot even be sure that the plan exists. Thinking it does may comfort us - but it cannot inform or enlighten us.
On the other hand, if you believe that knowledge of the divine plan is available to us, you face the contradiction in the nature of god and the nature of reality. So we face a tough choice - either god is not all-knowing, -loving and -powerful, or we cannot actually know what our purpose is - we can merely assume that they exist.
The second attempt at a solution is known as “humanism”, for no particularly good reason. It is similar to the divine plan in many ways, and falls victim to essentially the same counter-argument. Humanism is the theory that there exist transcendental values - that is, values which transcend any single culture or society. They are typically - but not always - enumerated as Truth, Beauty and Goodness, and our collective and individual purpose is the creation and enhancement of these values. There are two problems here:
Once again, I argue that there is no point in holding to something which reasonably has as much chance of existing as gremlins or the divine plan. And with humanism, there is not even the comfort factor of believing that an all-powerful god is looking out for you (at least while it's not smiting and ravaging, ala the Judaic god).
And this is the root of nihilism. It is the dawning of an understanding that life does not have a purpose or a meaning - it simply exists. Call it luck, chance, anthropomorphism or inevitability, there's no special reason why we are here.
It is the realisation that life has no purpose, that both divine purpose and humanism are the flailings of a drowning man. A drowning man who is grasping for straws that he would see do not exist, if only he could open his eyes. There are no superior values we should strive to uphold. There is certainly no benevolent God looking down and guiding us to some greater end. There is nothing. Ex nihilo nihilus.
This chain of though has led many to conclude that despair inevitably follows from a nihilistic world view, but this is confusing what nihilism says with what nihilism means. To the question “What is the meaning of life?” nihilism simply states that there is no purpose or reason why Life or life, occurs.. Yet this implies nothing about the purpose or lack thereof in my life - or your life. It concerns itself only with the purpose of all life - of all of reality in fact. Nihilism then, is freedom. It is freedom to choose your own purpose, freedom to make your own judgements, to stand on your own two feet and hold straight your own backbone.
It is freedom, because not only are the means ours to select, but the ends also are ours to choose. Freedom is not easy. Freedom takes work. It is a moral burden, but an honest one. Moral subservience is the easier path - but moral freedom is the superior one. Those who follow the creeds of divine purpose and humanism say “live this life, because you must.” Nihilists say “live this life, because you chose it.”