Often a person will ponder his place in the universe, the meaning of his life, and his power over his surroundings. Unfortunately, that sort of thought often leads to a sad conclusion, that an individual is completely insignificant on a cosmic scale. These are very dangerous thoughts, as their contemplation can lead to madness.
Science Makes You Small
Just think, you and I are but two of six billion living human beings, and countless dead men and women. Our planet is one of nine in the solar system, and one of the smaller ones, I might add. As well, our solar system is one of billions in the Milky Way, our galaxy. And, the Milky Way is but one of billions of galaxies, with trillions of planets in total.
Not only that, but it is widely believed that the universe came from chaos or "nothingness" before the big bang, and will eventually again deteriorate into chaos: on either a big crunch, where gravity causes all matter to become a single infinitely small point, or infinite expansion, where entropy increases to its limit. In either scenario, all life, all information, and all structure will be completely lost. Everything you or anyone does, says, thinks, or creates will eventually disappear.
Just to add insult to injury, another irony is heaped upon humanity. "What is the purpose of our lives?" some ask, often late at night, lonely, and/or somewhat drunk. Science tells us that there is a very simple answer: you exist solely to pass on your genes. Yes, your body, mind, and possibly even consciousness are merely manifestations of an effective genetic code trying its best to reproduce itself ad infinitum1.
So Why Bother?
So why do human beings continue to live? We already know that all our effort will have no effect on the universe, and will be lost in time. Many of us already accept that even one hundred years after our death, we may already be lost, our thoughts forgotten, and our genes hopelessly muddled.
Well, let's say that you understand this completely. That you believe these scientific facts to the deepest recesses of your soul. Now, a gun is put to your head, and you are told "if you tell me not to shoot, I will not kill you." Would you speak? Life is pointless, right? Avoiding the bullet implies effort on your part, but we know that all effort is meaningless. So why do you still want to avoid dying?
The reason why is because the human mind does not see infinity when it looks at the universe, it does not see its lack of power. Instead, human beings create their own conceptual universe from the physical universe that gives meaning to their lives.
A universe of your very own
Your universe exists based on what you experience. What you do not see, hear, taste, touch, smell, or otherwise sense, does not exist. You are not troubled by the six billion other souls in this world, since you will probably never meet well over five billion of them in your lifetime.
What is the most important thing in your universe? The answer is you. As far as you know, there is only one conscious being in the universe and that being is yourself. You have no way of knowing if any other human is actually conscious, or merely a shadow of your own mind. If the only way the universe exists is by the ways that we can experience it, and we cannot directly experience another human's consciousness, then no other concsious beings exist2.
How long does your universe last? For as long as you live. Barring ressurection or another "plane of existence," we can only experience the physical universe for roughly one hundred years. For this reason, what happens a million years in the future, or a million years in the past is of no real consequence.
Of course, these are just my personal opinions. I might be horribly, terribly wrong.
1 - For more information, read The Selfish Gene by Richard Dawkins. For a counterpoint view, try the Bible.
2 - Read "Them" by Robert A. Heinlein.