Definition of Athiesm.

[LongtimeAZDem]

6. Atheism is simply a lack of an affirmative belief in a deity

Atonal - lack of tone
Asexual - lack of sex
Apolitical - lack of political position

Theism is the affirmative belief in a deity, so atheism is the lack of such belief. The term implies no reason for the position, unless used with qualifiers, such as agnostic atheism.

It is quite possible to lack a belief in a deity for purely emotional or other non-rational reasons, though I would hope that they are less common.