This question would surely be answered in a different way by different people -- people probably have different reasons for it. Let me give you one possible reason:
One reason to be atheist is if you disagree with the idea of God that is put forth by the dominant religion in your area. For example, what if you think that it is inconceivable that a God would create people and then condemn many of them to eternal torment for being human? Would that not tend to make you atheistic?
So I think that one major reason would be if you cannot take the idea that your culture has about God.
That is the most ridiculous answer possible. This might surprise you but atheists really really really DON'T believe in your God. We are not running with our hands over our ears and we are not stubborn sinners who refuse to listen to the almighty. Because... are you ready?... We DON'T BELIEVE in your old, old, old, old, old, old stories. They are full of blatant inconsistencies, horrific immoralities, sappy fairy tales and hysterical voodoo nonsense.
So do NOT project your pseudo-morals on to atheists because we are not interested in them. We are not hiding from your big-white-bearded, psycho God, We are walking calmly through our lives, free from the medieval burden of superstition. If you want to get 'warm fuzzy comfort' by talking to yourself, that is your choice. But, please, do not arrogantly insist that your bronze-age belief-system is responsible for my modern behaviour. Your superstitions are completely irrelevant to me. Capiche?
Because they think that a relationship with God means not to do what u want any time in any place , they think that he will control them however its not true because if he wants to control us why would he let us be athiests from the very begining ? He has the power to make us believe in him and so strong but he`s a loving God that wants us to believe in him by love not force .
There is no simple answer to your question because all atheists are not a homogeneous lot and they have very different reasons for being atheists.
Atheism basically implies rejection of the belief in God/deities. Since God/deities have no physical or empirical existence, many men refuse to believe in God/deities. God or any such divine authority is a kind of the supernatural and there are men who do not believe in the supernatural because it is not subject to proof or verification. The rule of reason and the phenomenal growth of science and scientific investigation ever since the 18th century have consistently contributed to skepticism in the matter of supernatural faith in divinity. The insoluble problem of evil might also be another reason why men have largely turned skeptical towards the idea of an omnipotent, omnipresent and benevolent deity.