Atheism is the belief that no deity exists. Atheism is contrasted withtheism which, in its most general form, is the belief that at least one deity exists Atheism is not Humanism which is a non-theistic philosophical stance that emphasizes the value of human agency, and affirms the use of  science and reason as opposed to submitting blindly to tradition, authority or a supernatural source

The why someone comes reasonably to the conclusion that there is no god, is because they have founded their reasonng on one of the following assumptions:
1) If there were a god there would not be so much evil in the world.
This assumes a number of things: a) that what is happening is wrong, and is not necessary to  the  scheme of things. and,  b) that God can arbitrarily interfere with things, especially our free will Many atheists are former Jews who had believed that God would have prevented the Holocaust or individuals who are upset because "God" did not make things go the way they desired.
2) There is no proof there is a God.
The lack of proof is the result of assuming that any proof would be something not all ready in existence.  In other words, how does one "prove" there is a god?
3) Science explains everything
This is not true. Science does not explain how the phyiscal universe came to be in the first place. Nor does it explain how biological evolution can take place contrary to the laws of entropy. Either God or the Big Bang initiated the beginning of the Universe and science has falled to explain how the Big Bang came into existence in the first place (unless God created it)

Since no one can know that there is no god, we must conclude that devote atheism arises not from reason but from some other source.
 An individual's choice for atheism is the result of a failed relationship with their father or with their church.