Why do people become atheists?
This is a very good question; unfortunately, it isnt very easy to answer. There are perhaps as many reasons for being an atheist as there are atheists. What I mean by this is that the road to atheism tends to be very personal and individual, based upon the specific circumstances of a persons life, experiences, and attitudes.
Nevertheless, it is possible to describe some general similarities which tend to be common among quite a few atheists, particularly atheists in the West. It is, however, important to remember that nothing in these general descriptions is necessarily common to all atheists, and even when atheists do share characteristics, it cannot be assumed that they are shared to the same degree.
A particular reason might play a very large role for one atheist, a very small role for another, and absolutely no role whatsoever for a third. You can reasonably assume that these generalities may be true, but to find out if they are true and how true, it is necessary to ask.
One common reason for atheism is contact with a variety of religions. It isnt unusual for an atheist to have been raised in a religious household and to have grown up living with the assumption that their religious tradition represented the One True Faith in the One True God. However, after learning more about other religious traditions, this same person may adopt a much more critical attitude towards their own religion and even religion generally, eventually coming to reject not only it but also belief in the existence of any gods.
Another possible reason for atheism may originate in bad experiences with a religion. A person might grow up with or convert to a religious faith which they eventually find to be oppressive, hypocritical, evil, or otherwise unworthy of following. The consequence of this for many is to become critical of that religion, but in some cases, a person may become critical of all religions and, as with the previous explanation, even critical of belief in the existence of gods.
Many atheists find their way to disbelief through science. Over the centuries science has come to offer explanations of aspects of our word which were once the exclusive domain of religion. Because scientific explanations have been more productive than religious or theistic explanations, the ability of religion to demand allegiance has weakened. As a result, some people have come to entirely reject not only religion, but also belief in the existence of a god. For them, gods are useless as an explanation for any feature of the universe and provide nothing worth investigating.
There are also philosophical arguments which many regard as successful in disproving most of the common conceptions of gods. For example, many atheists think that the Argument from Evil renders belief in an omniscient and omnipotent god completely irrational and unreasonable. Although gods without such attributes are not disproven, there is also an absence of any good reasons to believe in such gods. Without good reason, belief is either impossible or simply not worth having.
This last point is in many ways the most important. Disbelief is the default position no one is born having a belief. Beliefs are acquired through culture and education. It is not ultimately up to the atheist to justify atheism; rather, it is up to the theist to explain why belief in a god is reasonable. In the absence of such an explanation, theism should be regarded as irrelevant at best, but more likely irrational.
Thus, a better question than why are people atheists would perhaps be why are people theists?