As Richard Dawkins points out, I have no obligation to explain why I am an atheist, it's for those who believe in a god to supply evidence.
Atheism, religion, science and ethics are linked:
Religion provides a simple world view based on the existence of one or more gods. Gods are super-beings who are unscrutable, far superior to humans and endowed with supernatural powers. Such a world view starts from the axiom that humans will not be able to understand the world around them. There is no further argument possible: one lives "by the book".
Religions are respectable, their leaders have considerable political influence. Sects are viewed differently, but why?
Pope Benedict XVI said in 2006: "don't make your own 'do-it-yourself' religion". Perhaps, but was Christianity not a do-it-yourself sect in the beginning? Why should I not be allowed to start a new one now? What is so special about four or five books written almost 2000 years ago, many decades after the "facts"?
Also in the year 2006 Prime Minister Iyad Allawi backed the referendum vote on Iraq's constitution, noting that unlike the Koran, a constitution can be modified. What is so special about a book written 1400 years ago?
In 2006-06 an Afghan citizen was nearly condemned to the death sentence for having switched religion. You can join, but you cannot leave. The same death sentence is pronounced in the Bible, the Koran and the Torah for the same act of apostasy.
In a recent interview (2006-08) with the Italian popular science magazine "Newton" I said this:
Q.: DO YOU BELIEVE IN GOD? (or alternatively, WHAT IS YOUR RELATIONSHIP WITH FAITH?)
No I don't. Religion is a superceded explanation of the world around us. From early animistic ideas it became a well-organised set of beliefs attempting to explain physics as much as psychology. It comforted people by relegating the unexplained to a "God".
Religion lived alongside autocratic government. The Renaissance discovered that government is better by democratically constructed laws, changing laws when necessary through elected governments. It also discovered that explaining the world is better through theories tested by experiment against reality, changing theories that fail the test.
There is no place for dictators once you have democracy, there is no place for blind faith once you have the scientific method.
Clerics and spiritualists don't like people who think for themselves. They try to separate the scientific method from everyday life. But the experimental method checks with reality. We use it all the time: you gather evidence and check it before deciding.
Thinking independently, checking with reality and remaining intellectually honest is in direct conflict with religion.
Right up to this very day, wars are caused by stubborn beliefs in ideas that are wrong, most of them of religious origin. Religion is a nasty side-effect of our imperfect brain structures.
For that interview I was allowed only 200 words, so I had to be brief. I would have liked to add:
Yes, almost all of us use the scientific method in everyday life, irrespective what culture we grew up in. Common sense tells us it is better to check before taking a decision. People don't normally buy cars on "faith". Those who do usually end up with the wrong product.
Time does not allow checking everything and hence sometimes we regret decisions with a sigh of "Had I only known..."
Common sense is just a first level of the scientific method. Applying common sense is formalised in science.
Atheism however does not by itself give me a set of values and rules by which to live. The criticism most often used against atheism is that crimes were committed by atheists, notably in the communist regimes. The link between communism and atheism is tenuous: any belief/value system becomes dangerous when it becomes dogmatic. But there are links between behaviour patterns and belief systems.
For a more detailed set of points and arguments, you can consult "The God Delusion".