Hope-- Great question!
First of all, let me say that if beleiving in something makes you happy and gives you meaning in your life, then by all means--believe.
So if one does not believe in God, and believes that they are a product of mere chance, how does one find meaning in their existence?
The first thing for me is, I don't create a reality for myself because of how it makes me feel. Reality is reality, whether it makes me feel good or not.
I used to think I could never be happy or moral unless I believed in God. But I always questioned anyway (I suppose I was more interested in my idealistic view of the objective 'truth' than in being happy or moral lol), and my questioning has lead me to conclude that an all powerful omniscient creation-spirit is just like any other religious belief in human history, with no more relevance to reality than Zeus or Thor or witchcraft. I realized that religious beliefs are unfounded and only survive because they serve as explanations where, psychologically, ANY explanation would be preferable to none (even an imagined one); because of how they make us feel; and because they provide stability and conformity in our societies. What happened after this line of thinking completely surprised me.
I started thinking of "God" in different ways. I started thinking that, perhaps, the transcendental "God" that many people believe in in Western culture is really just a personification of the cosmos itself, and how individuals and society regard it...with awe, humility, and a desire to carry out its "will"/be in harmony with it.
Beleive it or not, but when as realized I was an atheist, I began to feel free. It was as if I was seeing the world as it was for the first time. Suddenly I felt like my mind was out of the Matrix or something...I could suddenly look at the world and think about it and respond to it without all the a priori baggage that I had previously struggled to reconcile. I could suddenly think for myself, and question things that I had previously thought it "wrong" to question. You see, that's what religion does...it makes you feel guilty for doing that which is perfectly normal--questioning. Finding your own answers.
To my own surprise, I didn't become depressed. I realized that we can choose how to feel about something...but we can't choose what the something is. I want to know what the universe is, but I've already made a choice as to how I feel about it--call it an atheist's faith, if you will.
That's right, you heard me! Faith. I think faith is perfectly acceptable when it comes to emotions. After all, outside of 'me' there is no 'correct' way to
feel about things. How I feel doesn't have to be logical at all. I think of my faith as acceptance. Acceptance that, ultimately, I don't know, and that's okay. Acceptance that, even though it doesn't all make sense to me, perhaps it wasn't meant to, and that's okay too. Acceptance that the universe is greater than I am, and yet it would never be the same without me, either. It's a stoic, humble, attitude towards the infinite cosmos and my place in it, and I must say I get a great deal of satisfaction and inner peace from it.
I am in complete awe of this vast, incomprehensibly complex cosmos that we live in. It's a universe in which matter condenses and forms gaseous ringed planets, huge red giant stars, and spectacular spiral galaxies; in which organic matter forms in a stunning variety of atmospheres and oceans and climates, and develops into cells and life forms so bizarre and beautiful that I am reminded of thousands of images of ants running, sharks capturing prey, gorillas nursing their young, flowers blooming, and groups of flamingos taking off, all racing through my mind, as if cued to the music of 2001: A Space Odyssey.
Are we no more than organisms, no more than cells, no more than matter, you ask? To that I say, what do you mean, "no more"?! Do you require something more beautiful, more complex, more utterly inspiring than the invisible interactions of an incomprehensible number of electrons and protons with positive and negative charges, the incredible complexity of a single strand of DNA, the intricacies of our brain's 'software'--our cultures, our languages, our capacity for imagination? Who needs to believe in any 'extra' stuff to appreciate the chaotic beauty of our cosmos?
Compared to some advanced alien race, we are like a colony of ants. And yet, can you honestly look at a colony of ants, and not appreciate them for what they are? Would you honestly say to them "Hey, you ants! You aren't important, and you can't accomplish anything!" This, despite the fact that they have a complex society and create structures we humans could never dream of building on a similar scale. I say, go forth, little ants! Take part in your ant societies, do what it is that makes you ants; and multiply, because the universe is just that much more interesting and beautiful with you in it than it would be without you.
I know I've been ranting...but I really feel passionately about the meaning of life and so forth.
Who are we? What are we? We have only begun to discover the answers to these questions. And yet, somehow, the fact that these questions have not been definitively answered only makes the journey that much more exciting.
*edit* I think this about sums it up: "Beauty is truth, truth beauty" ~John Keats