Lets try it this way. I am an atheist. This simply means that there is not a god in my life. I don't walk around all day not thinking about god, but rather doing stuff without a god belief or god thought or god-based reality. I have no reason to think that there is a god, nor do I feel a need to prove to myself that a god does not exist. My life lacks prayer, church, church potlucks, church committees, church sermons, etc. but otherwise its all there. I eat (again, no prayer), sleep (no prayer), work, play, have friends, etc. All without a god.
I'm going to go out on a limb and guess that both of agree there are no leprechauns. So, each of us gets up every morning, goes through our day, gets home at night and goes to bed without giving a thought to leprechauns. Do you feel you should have to justify, in any way, not believing in leprechauns? Do you have the needed proof to justify your position the next time someone asks you about it? Of course not.
Now obviously religion is an important part of many peoples lives, and hence those of us that reject it at times confound or anger those who so deeply believe. Regardless of the specifics of their beliefs, however, none of them can show me any of their gods. Each is invisible. Each is unknowable in any normal human way. Each requires some variation on the "faith and belief" theme.
We atheists simply look at the gyrations required to justify any given religious belief and conclude that there is nothing behind the curtain, so to speak. Since no religion can prove itself to be true and real, some percentage of the planet is going to shrug its shoulders and say "I doubt its really true that there is a god", and many of those people agree to be referred to as atheists.
If nobody believed in a god, nobody would have to be an atheist.
Those who do believe in a god are apparently perplexed at the idea of not believing, but it would help a lot of they could at least be accepting of the idea. Since no christian or member of any other religion can point up into the sky (or anywhere else) and say to me "Look, there he is. He's waving right now", no religion can meet my standards of proof. All religion can show me is a back-story, a conveniently supportive version of history and a book that contains the "truth".
None of this would matter much if each and every version of belief could keep its specific moralities and rules and expectations to itself and let society in general come up with something "average" the generally met the needs of everyone without stepping on the toes of anyone. However, many special interest groups, including the religious, undertake nefarious projects, like trying to get creationism taught in schools or wanting their god to be acknowledged at on a regular basis by everyone, regardless of religion or non-religion. I learned the Pledge of Allegiance in 1953 as a first grader, and had to learn a modified version in 1954 because suddenly America was "under god". Now thats not a big deal to me though I think its pretty cheeky of the religious to shove stuff like that down my throat. But then we have to ask where it will stop. At what point will I, an atheist, eating at home alone, be required to say a prayer before I eat dinner? Obviously nothing has gone that far yet, but how do I know that if we don't complain about school prayers and such that it won't eventually get to that.
I am an atheist, and I feel that I should be able to freely express my interpretation of reality without fear of reprisal. I do not use atheism as an excuse to be a bad person. I don't go door to door promoting atheism. I don't undertake efforts to get all church doors boarded up or to enact laws that would keep children out of sunday school. I'm a pretty nice guy. But I do not see any evidence of a current or past god ever existing, and I want to live my life free to assume that I am right. If god doesn't like it, he should come to my house and have a little talk with me. He knows where I live. Right?
I was watching a video today on YouTube with Ann Druyan, wife of Carl Sagan when he died. She told the story of how her father went home from college to tell his father that he was an atheist. His father was an incredibly devout Jew. When er father told his Dad, his father responded "Well, the only sin would be to pretend".
If all believers could be as accepting of those who disbelieve, we here on this site would be so bored the site would shut down. That so many are vocally against believers means that we have to band together in various ways to save not only our freedoms but perhaps also our lives.
We don't need proof that there is no god. We're not the ones claiming there is. And our individual reasons for rejecting the notion are nobodies business but our own, unless we choose to reveal it. I'm sure everyone is tired of hearing it, but I've been an atheist now for 50 years. I'm happy, I'm healthy, I'm doing just fine. It all works for me.
Its fine that you're asking the question. If this doesn't satisfy you, then you probably didn't want an answer anyway.
(Note: If you're big into the hell thing and can't understand why I would dismiss the fear of fire and brimstone stuff, bear in mind that I don't have any good reason to think it exists. I've been given the Jesus talk numerous times in my life and if I'm wrong and there is a heaven and hell, I've nobody to blame but myself (and the incompetent god who couldn't get his message across to a relatively literate adult). Don't you worry yourself about any atheist who you think might end up in hell. Its not your problem.)
I was supposed to go to bed hours ago but I got into a long phone conversation then found your question here and just had to answer it. So now I'm off to bed. Without prayer. Don't worry. I'll be fine.
P.S. Thanks for inspiring me to learn how to spell "leprechaun". It might come in handy some day.