I'm phrasing this as a question, but honestly it's something I hear as an accusation ("How could you turn your back on God?") or as an assumption, sometimes implicit and sometimes stated openly. ("Romans 1–3 makes it plain that the knowledge of God is written on our hearts—every human being knows there is a God and every human being is 'without excuse' (Romans 1:20). So there are no true atheists." -Ken Ham) However you phrase it, it's a very common conception: this idea that becoming an atheist, like becoming a Christian, is a choice.
I don't think that's true. I think it's untrue for many Christians, maybe even most Christians. But I'm sure it's untrue for a great many people who started as Christians, and later lost their faith. I know it's untrue for me.
The problem is, I don't think the opposite is true, either. So, I don't want to just say, "It's not a choice," because that makes it sound like unbelievers have no volition - as if we're hopelessly at the mercy at forces beyond our control. That isn't really how it works, either.
It's more complicated than that.
Mainly, I don't think that the language of choice is a good way to talk about faith at all. When it comes to the loss of faith, it's even worse: it isn't just insufficient, it's misleading.
I've said this in various forms before, but - as far as I can see - the loss of faith isn't a decision in and of itself. It's one possible conclusion to a much larger process.
It starts with questioning. There may be some particular slight or injustice that acts as a catalyst, and it's easy to focus entirely on that. ("She just had a bad experience with that one church." "He's just angry at God because his puppy died." "If her Youth Minister had been able to answer her questions, she wouldn't have left.") Or, there may not be any obvious cause; a lot of people start into this process precisely because they were trying to take a deeper look at their faith, or simply because they were trying to put things together and couldn't quite get the pieces to fit. But even if there's one particular moment or incident that seems to have started someone on the road to disbelief, it's a mistake to put too much focus on it. It might have been the trigger, but almost certainly isn't the cause. Again: it's more complicated than that.
So: it starts with questioning. The questioning is the important part: what a person is wondering about, or troubled by; what they find as answers; how they sort through possible answers and reach their conclusions. Everyone, believers and atheists alike, is doing the best they can with the information they have... but we don't all have the same information, and we don't weigh particular pieces of data in the same ways, and sometimes there are other factors as well. (It's possible, for example, that a bad enough experience can basically spoil someone for a particular faith - it's possible for someone to have personal reasons that make them unable to take part in something, without necessarily believing that the thing itself is evil and wrong. Presumably God, if He is as all-knowing and all-loving as advertized, would understand and even sympathize.)
So: after the questions, there's a period of evaluation. Some people find answers. Some don't find answers, but retain their faith anyway - faith being, for them, the substance of things hoped for, the evidence of things unseen. Some lose their faith in particular doctrines, or churches, or denominations... but retain a fundamentally Christian faith, either changing denominations or becoming "Christian but unchurched" or "spiritual but not religious".
Some change religions entirely, but remain religious. They become Wiccans, or Buddhists, or... almost anything, really.
So atheism isn't the only possible result of this process. I suspect it isn't even the primary result of this process. Atheism is where you end up when you conclude that every religious, spiritual, or supernatural way of looking at the world just doesn't work for you. And even then, it isn't necessarily an end-point. Some people leave religion, and remain unbelievers for the rest of their lives. Others pass through a period of unbelief, and eventually return to some sort of faith - either the religion of their birth, or something similar, or something else altogether.
That's what I see. That's what I hear from other unbelievers. The idea that someone would "choose" not to believe sounds... odd. Off-key. It seems like the sort of thing that only works as an answer if you're asking the wrong questions.
The idea that unbelievers are "suppressing the truth in unrighteousness" (because everyone knows that God exists and that He looks exactly the way that particular strains of American Christianity describe Him) is worse. It says that I cannot possibly be what I am. It says that I'm either lying, or I'm delusional; possibly both. I suppose that might be reassuring to believers, but it's utterly useless to me. I know I'm not lying, and I really have no choice but to assume that I'm not delusional. Once someone says that to me, the conversation is pretty much over; there's nothing I can say to that, and there's no reason to listen to it.
Atheism isn't a choice, and asking if (or assuming that) it is... is a fundamentally misguided approach to understanding it.
 I've seen arguments that everyone is born as an atheist, but I don't think that's entirely true; I think the truth is a bit more complicated than that. People are born without any specific religion, but the tendency towards religious belief in general - towards belief is some sort of Unseen Forces That Shape Our Lives - seems to be wired into the species.
 Also, if I am delusional - or even just wrong - a loving God would understand and accept that.