If you believe in only what you can prove repetitively (ie. naturalism, or belief founded in science) then the reason for existence would be death and sex, this is proven through billions of years of each organism dying and providing sought after nutrients, energy and life for future organisms. This is of course obsolete when you realise that after billions of years the Sun will expand and exterminate all life on Earth, and even more obsolete when the universe decays into comparative nothingness. As a result basing your existence on what can be repetitively proven is pointless since live will cease to exist eventually anyway.

As such the reason for existence for higher thinking organisms like humans must be more than what we can prove repetitively in order to encourage us to continue breeding at a rate that maintains our population. Such beliefs provide hope in time after death through promotion of stability in life.  these beliefs may come in such forms as government stability to promote a future that is unproven, or more complex that exist out side the normal realms of science and therefore either unrepeatable, unforeseeable, or simply unknowable.

Whatever the belief, the simple fact is that a true reason for existance has a simple but vital rule to work efficiently. It must be unable to be disproven. By disproven it is not to say that you can not repeat the conditions, but that you can not say it does/did not happen. The majority of major world religions fit into such a category.

No one really knows. I suggest you read several books on comparative religion and books that cover the major philosophers from Greece to current times. Between them these should provide you with a grounding in what various people have thought about this subject. The fact that they all disagree, should give you something to ponder. Eventually you can use  the information to synthesize your own ideas about these questions.

This is a philosophical question, by which I mean a question that can only be answered effectively within the scope of philosophical disciplines. Nothing compels you to care about those disciplines of course, but if you neglect them, then it is hard to see how any answer you get could mean anything to you or anyone else. Of course, if you do accept meaningless answers they might satisfy you tremendously, and this would mean that your question has been a successful attempt to get some peace from the nagging of questions that you were not equipped to understand. In every other way however it would be just as good as if you had never asked the question. Certainly it would not put you in any better position to discuss the matter with anyone who was not equally satisfied with meaningless answers, and that is a rather ignominious capitulation when what one has asked about is "meaning"...

However, since you have asked the question, I assume that you would like some sort of meaningful answer that would give you a better understanding of what you think it is that you have asked. To achieve that you will need to do a lot of ground work. If thatwere not so then these questions would not have been so hard that they have plagued hundreds of generations of philosophers and would-be philosophers. Not everyone who has asked the question futilely in the past was necessarily stupider than you or I.

As for which ground work you should do, I do not recommend that you begin by studying the history of the question. Such a study is long, tedious, and not clearly relevant to what you want to know. In fact that history is largely a history of futility. If such things interest you, then you might do better to go back and study the history only after you have achieved some sort of coherent view of the concepts, and have either come to a conclusion or what seems to you a prospect of a conclusion or some reason why you should not expect ever to find a conclusion.

Perhaps you should begin by examining some of the material on the Internet, such as Wikipedia and the Stanford Encyclopaedia of philosophy. Make liberal, but cautious, use of search engines (such as that big popular one). Look up such subjects as semiotics (the study of meaning and communication), formal logic (without a few of the concepts of formal logic, it is not clear how you are to understand what some aspects of your question mean), ethics, and origins. And pay particular attention to the meanings of "meaning" and "why".  (Very treacherous words, those!)

Your next line of study should include some of the facts about your own nature and of the world about you. After all, fundamentally that is what you are asking about, and before you understand in what ways this is so, it is no good asking anything about it. Read some of the sounder popular books on evolution. Try "Darwin's dangerous idea" by Daniel Dennett. It is a remarkably complete and balanced introduction, bearing in mind that Dennett is a philosopher rather than a biologist. Then try some of the Dawkins works. Probably a recent edition of "The Selfish Gene" would make a good start. "River out of Eden" might be next, and "Climbing Mount improbable" is a fine mind extender. If you still are a glutton for punishment after that, you could try some harder work, say by John Maynard Smith et al. "The Origin of Life".

Continued on next rock...

Your question presupposes that there is a plan for the universe and you are searching to find your part in delivering it.  This is the foundation of many belief systems and the basis of abuse by some spiritual leaders in imposing their own agendas in the guise of their chosen deity's. By asking a loaded question like this you remove the freedom to explore what your existence really means to you.

If the universe is, as many believe, governed purely by physical laws and cause and effect, then it is entirely up to you what meaning you give to your existence.  Just make it count.

My mother once challenged me to explain, if I was so clever, why we were here.  I pointed out that in my particular case she was better informed than me.

The search for a reason for our existence is not logical.  Every reason given opens the gate for another question at a higher level.  It's like a Christian asking his vicar, "I know our ultimate purpose is to please God, but does God worry about why he exists?"  The ultimate question is, "Why is there anything?", and even if that was answerable, you might still ask, "Why was there an answer?"

I suspect that the logic of cause and effect that we find perfectly sensible in our everyday lives is not up to the task you've given it.  I hope it might be possible to be able to say in some mathematical language that it is inevitable that certain things cannot, in any situation, not exist.  Unfortunately, I am almost certain that such a statement will not sound logical in English.