Mostly, we didn't. This is a common misconception among Christians... they assume that everyone believes in God, and that people who pretend not to believe must have had a traumatic experience that made them hate God (see below). Commonly it is assumed that an atheist must have, say, prayed to God and not been answered, or had a loved one die, and then renounced God in anger.
In reality, few people come to their atheism that way; and those who do usually don't stay atheist for very long. In the majority of cases, you'll find that atheists have thought very hard about their belief in God, and found that it just doesn't hang together logically. A great many atheists were raised in a religious household and decided, after much inner struggle, that they just couldn't continue to take their faith seriously. A few were brought up in atheist households and taught to think about the world and question conventional wisdom.
Ironically, many evangelists use their own stories of traumatic events as a way of convincing people to find religion. Often you will hear stories of how a person had "hit rock bottom", was perpetually drunk and unemployed and had no hope for the future, and that's when they found the Lord. While they dismiss us by saying that we must have become atheists for dramatic emotional reasons, they use the same techniques to recruit new parishioners.
Posted in: About atheism