There are a variety of stories I've heard and read about how people became atheists. One I've mentioned before is Penn Jillette's story of how reading the Bible prompted him to become an atheist (which I call a case of Biblical atheism). Richard Dawkins, if I recall correctly, claims to have intuitively come to the conclusion that no gods exist while fairly young rather than having undergone a deprogramming or deconversion experience.
Mark Bauerlein, on the other hand, had a sudden realization that convinced him that there was no god at all, that such supernatural and transcendent notions were utter fiction. A sudden realization that carried with him for 3 decades of his life. Holly Ordway never had a sudden realization, but rather gradually came to be an agnostic as a child and then an atheist after an irreligious upbringing.
What's interesting is that in none of these cases did any of these people become atheists because they were mad at God. While it is a fairly common thing (at least in the U.S.) for a certain subset of zealous Christians to believe that atheists are all mad at God, it's simply not the case that all atheists reject the concept of God because they have built-up repressed anger from childhood because of a tragedy. And the angry atheist is going to be less and less common as more and more children are raised by atheist parents who don't set them up for disappointment by telling them, "God will never let anything bad happen to you."
So the trite trope of the angry atheist professor in the film God's Not Dead is understandably appealing to those who think of atheism as just a childish emotional response, but it doesn't have the virtue of being straightforwardly true. Which is not to say that there aren't people who reject God because of tragic circumstances about which they are angry. One young man I encountered stopped believing in God fairly young after his dear friend died of cancer, and he was quite obviously angry about it.
He believed firmly that a loving God would not let young people suffer and die, and that any God not loving in the sense he meant it was a bad God and not worth worshiping or trying to have any kind of relationship with. He was wrestling with the Problem of Love, as most of us do at some point. Sometimes we call it the Problem of Evil, but on a personal level, it's the Problem of Love, because we no longer believe that God could love us and allow such suffering to befall us.
And this is something we should not at all be aghast at when we encounter it in our atheist friends and family members. Christians of all people should be wrestling with the Problem of Love. We have been shown by the death of Christ on the cross that Love Himself suffers and dies, that God sent His only begotten Son to die for the sake of love. The Christian who has not yet asked, "My God, my God, why have you abandoned me?" as Jesus did will almost certainly ask it once before they die themselves.
No Christian who has read the Gospels should look down upon the atheist who is wounded deeply by death and suffering and the sins of believers, believing that God has abandoned them. Of all people, the People of the Cross should be compassionate with those who cry out that God is missing from the circumstances of their lives.
And of all people, the People of the Cross who often fail to live up to the commandments of the one who loved us unto death should know that in each moment when we do not show people the love of Christ in the midst of suffering, the less believable the love of Christ becomes for those who witness our lives. If we abandon them because of their atheism, then are we not demonstrating once more that love fails in the midst of struggle and suffering, that there is no resurrection of love?
If we as Christians would seek to understand why people become atheists, we should look at our own lack of sincerity about our religion, our own doubts and anger, our own struggles to understand the connection between love and suffering, and our own actions which fail to show people that love and suffering are inextricably bound up with one another.