Atheists often get told they've become so for irrational reasons. One of the most fatuous claims I've heard (and I'm sure I'm not unique), is that I've become an atheist because I love to sin. In short, I really know there is a god, but I'm just pretending there isn't because I prefer some kind of sinful life.
This betrays almost no prior thought. If anyone really believed there was a supreme deity with the attributes ascribed to it (love, unconditional mercy etc) then there isn't really any motive to pretend otherwise. Especially when you'd be willingly choosing an eternity of presumably, heinous torture (as many theists are fond of reminding me). Does this actually make sense as an argument? I know there's really a god but I prefer infinite torture to some kind of eternal paradise? It's got to be one of the dumbest arguments I ever hear.
Especially, as I should point out, that if you are a theist, you're not banned from sinning. If you're a theist, you can sin (you have to, it is in your nature because of original sin or some other imperfection). And you still avoid hell. So you get it all. Atheists are smart enough to figure that out. There's no reason to pretend. We genuinely have no belief in the gods that have been supplied to us.
For the most part, people become atheist for a very mundane reasons. There is no credible for gods, and the whole concept often sounds wildly implausible.