Who the hell are we? Why are we here on this planet? Why did we evolve? What is the reason of being alive here right now? And why do we feel that urge to mate (put in "decent" terms)? So many questions. Some philosophical, some existential, mostly asked by people lying on grassy plains looking upwards to the starry heavens or/and while stoned and listening to Pink Floyd. Also mostly asked by people from rich countries who have things other than making money on their minds and lot of time on their hands. But I don't think there is a human being who has not asked himself/herself this question at least once. Even then, we seldom realize how much by luck and chance we are here alive today, that we actually came about to be.
How much do we take ourselves and our lives for granted? Do we realize that for each ones of us who are alive today, there are countless millions of billions of organisms who failed to make it? Do we realize that as living beings we are 3.8 Billion years old? Hell, do we realize anything at all? Do we remember or again, realize, all the pain, agony, death and suffering all the billions of our ancestors have undergone only and only that we could exist today to feel the warmth of the sun, the caress of the wind and the traffic jams of Bangalore? Do we? Despite all this, we continue to squander our lives away.
P.S.: This applies even for creationist nuts. Also when I mean "your", I mean I, me, included.
A Triumph, an Unabashed Unsung Victory of Nature
We all know about the "winner sperm" adage, that you exist because one half of you (that is right, the sperm is only one half of you - the other half was the egg) beat millions of other sperm to fertilize the egg, which ultimately became you. But there is more to that. You (and I) are a living, breathing 3.8 year old success story of nature. Just like that Sperm, you are one of those one in a million zillion living beings who were successful in making it this far from that first single-celled organism which evolved and swam around the Hadean Oceans 3.8 billion years ago. A triumph of Nature. I cannot put it better than Bill Bryson:
"Consider the fact that for 3.8 billion years, a period of time older than the Earth's mountains and rivers and oceans, every one of your forebears on both sides has been attractive enough to find a mate, healthy enough to reproduce, and sufficiently blessed by fate and circumstances to live long enough to do so. Not one of your pertinent ancestors was squashed, devoured, drowned, starved, stranded, stuck fast, untimely wounded, or otherwise deflected from its life's quest of delivering a tiny charge of genetic material to the right partner at the right moment in order to perpetuate the only possible sequence of hereditary combinations that could result -- eventually, astoundingly, and all too briefly -- in you."
— Bill Bryson (A Short History of Nearly Everything)
Also, as this Web Comic, Abstruse Goose says here:
You are the end result of billions of years of sacrifice, despair, death and amorous relations. Your life represents the culmination of an unbroken series of evolutionary victories from the very first single-celled organisms all the way down to you.
Your life is not one that came around by accident, nor is one that can be taken for granted. Each one of your ancestors has fought, escaped and survived onslaught from nature, predators and competitors alike to survive, find a mate and bear offspring to keep the line alive, which ultimately resulted in you. You can trace your ancestry back in a straight unbroken line, from your parents, grand parents, great (n) grandparents, forgotten humans, proto-humans, apes, mammalian creatures, amphibians, ancient sea-dwelling creatures, trilobites, even plants and then complex celled organisms and finally single-celled organisms, back to the very first organism at the very dawn of life, with many sub-lines breaking off and rejoining at infinite number of points with thousands of branches perishing without leaving any trace, only your line surviving. For all those lines that perished, ending at individuals whose existence was terminated by disease, war, famine, murder, being eaten or just killed, your ancestors survived, found a mate and reproduced. Yes, in an unbroken line, you are 3.8 Billion years old.
The Story of your Ancestors, Written in Blood - for You.
Your line might contain kings and chieftains, warriors and robbers, saints and villains, heroes and thieves. Your lineage also contains individuals who were offspring between brothers and sisters, fathers and daughters, women raped and thrown away, prodigy of slaves and harems, of unknown fatherhood and countless orphans. Imagine how many, many of your ancestors would have escaped certain death by a hairbreadth, how many might have just survived to produce offspring. Behind your existence today are a billion unknown, unwritten and forgotten stories of death and despair, gore, horror and survival, of war, famine, drought, migrations and struggle, of incest, rape and amorous relationships, of glory and horror, of sacrifice and tears, of cruelty and kindness, of heroism and betrayal. Tales of history that have made it possible for you to be here. Still, these lineages that are 3.8 Billion years old are being voluntarily terminated every day. What did all your ancestors suffer for? What will you tell all of them? They all lived and died for You!
Tales like that of your great(n) grandfather who got killed while bravely trying to fight off a pack of wolves to save his mate and their child, A great(n) grandmother who commanded all her will to stay alive just long enough to deliver her baby alive before dying of complications from childbirth, great-great(n) grandparents who traveled hundreds of kilometers braving the harsh forces of nature to escape from oppression/poverty, of grandmothers who were abducted, raped and left to raise their children on their own, stories of tears of children separated from their parents, of a furry mammalian creature which escaped the monstrous predators of the Mesozoic while trying to gather insects to feed his family, an ancient fish which escaped slimy predators by a hairbreadth or simple celled organisms which survived the Lava that suddenly erupted on the Protozoan sea floor. And especially tales of our grandmothers!
But each and every one of them survived and passed on only after making sure that you will be born millions of years later. If any of these lives were to be snuffed out untimely, you would not have been here. Coming to think of it, your life is as fragile as a 100 GB file download over a Tata Photon internet connection. The odds are stacked only too heavily against you. And imagine, the right combinations at the right time enabled you to become Nature's most superior life form - and not a worm under a log in some murky swamp or a Lichen on a rock in Antarctica. Which takes us back to the all-important question - "Why are we here?"
The Crowning Glory of Creation!!
Each one of our ancestors battled and braved everything in their short lifespan to forward their kind. Those days there were no arranged marriages, social security, super markets or multi-speciality hospitals, so the only way to propagate your kind was through natural selection. If you were diseased or weakly, then tough luck, you had to be weeded out. That was the law of nature. Despite everything, you are still one of the healthiest in your line, ever. Also, you are probably related to everyone else on the scale of n-th cousins. Coming to think of it, the only reason why every one of our forebears were alive as only so that they could keep the line going. And that reason applies to us too. We exist to ensure the survival of our species. Even Count Dracula tells Van Helsing so in the movie. Nature wants us to. Nature still governs us. Hence the feeling of "that". We exist to find our mate, have sex and forward our line. That is the rule of Nature, the reward for which will be that all-consuming feeling of awesomeness, happiness and tears, of having achieved what you were created for, when you see your line, the next of Nature's victories taking shape... Bask in the glory of the realization that you are one of Nature's, of the Universe's biggest achievements, the crowning glory of creation! There you have the answer to life's biggest question, The Reason for our existence.
In other words, each and every single one of your ancestors were successful in getting themselves laid. So do not despair! :)