You choose to disbelieve in God (and you should choose to believe in God).
I frequently hear the claim that atheists choose atheism, usually for some morally blameworthy reason like a desire to avoid taking responsibility for their sins. My response is basically the same every time: You may not believe me, but I didn't choose any such thing, and I can't just 'choose' to start believing. Maybe you can, but I can't. I do not believe in any gods. Evidence would make me believe in some god, but all the playacting in the world isn't going to change that.
Why? Because belief itself simply does not appear to be a matter of will or choice. A real problem with this idea of "voluntarism" in beliefs is that an examination of the nature of holding beliefs does not lead to the conclusion that they are very much like actions, which are voluntary.
When an evangelist tells us that we have chosen to be atheists and that we are deliberately avoiding belief in a god, they are not entirely correct. It isn't true that one chooses to be an atheist. Atheism especially if it is at all rational is simply the inevitable conclusion from available information. I no more "choose" to disbelieve in gods than I "choose" to disbelieve in elves or than I "choose" to believe that there is a chair in my room. These beliefs and the absence thereof are not acts of will which I had to consciously take they are, rather, conclusions which were necessary based upon the evidence at hand.
However, it is possible that a person may wish that it not be true that a god exists and, hence, has directed their research based on that. Personally, I have never encountered anyone who has disbelieved in the existence of a god based simply on this desire. As I myself have argued, the existence of a god doesn't even necessarily matter rendering the truth emotionally irrelevant. It's arrogant to simply assume and assert that an atheist is being unduly influenced by some desire; if a Christian sincerely believes it is true, they are obligated to demonstrate that it is true in some particular case. If they are unable or unwilling, they shouldn't even consider bringing it up.
On the other hand, when an atheist argues that a theist believes in a god simply because they want to, that isn't entirely correct either. A theist may wish it to be true that a god exist and this could certainly have an impact on how they look at the evidence. For this reason, the common complaint that theists are engaging in "wishful thinking" in their beliefs and examination of evidence may have some validity but not in the exact way that it's usually meant. If an atheist does believe that some particular theist has been unduly influenced by their desires, then they are obligated to show how this is so in a particular case. Otherwise, there's no reason to bring it up.
Instead of focusing on the actual beliefs, which are not themselves choices, it can be more important and more productive to focus instead on how a person has arrived at their beliefs because that is the result of willful choices. As a matter of fact, it is my experience that it is the method of belief formation which ultimately separates theist and atheists more then the details of a person's theism.
This is why I have always said that the fact that a person is a theist is less important than whether or not they are skeptical about claims both their own and others'. This is also one reason why I have said that it is more important to try and encourage skepticism and critical thinking in people rather than to try and simply "convert" them to atheism.
It is not uncommon for a person to realize that they have simply lost the ability to have blind faith in the claims made by religious tradition and religious leaders. They are no longer willing to shut away their doubts and questions. If this person then fails to find any rational reasons to continue believing in religious dogmas, those beliefs will simply fall away. Eventually, even the belief in a god will fall away rendering that person an atheist, not by choice but instead simply because belief is no longer possible.
Read a more detailed examination of Beliefs and Choices.