Children are like sponges; at an early age they absorb everything around them. That is why major events in their childhood could haunt them as adults.
Since LOST was a character study, what were the first major impacts on the main characters?
For Locke, it was being told that he was "a miracle baby" but abandoned by his parents. It was this paradox that led Locke on a futile quest to find a family.
For Kate, it was stealing in the rural general store and getting caught. But the consequence was that she got off (a stranger paid her debt) which led her to a life's belief that she could get away with anything; no accountability or responsibility for her actions.
For Jack, it was a school yard fight. He intervened with bullies were taking on a student. Jack got beat up and when his father told him he was stupid and not a leader, it put a permanent scar on Jack's self-esteem in that he could never live up to his father's expectations.
For Hurley, it was the day his father left. He was a happy kid until that event. It traumatized him to the point of eating to hide his pain for he believed that he was the cause. This would lead to a life of self-blame, isolation and self-doubt.
For Sawyer, it was hiding under his bed hearing his father kill his mother and then himself. This turned Sawyer into a vigilante, on a quest to find the con man who destroyed his family. And in this quest, he was twisted into the thing he hated most: he turned into a murderous con man.
For Ben, it was his birth that caused his mother's death. His father constantly blamed Ben for killing his mother. He was constantly told he was a monster. As a result, he kept his emotions inward to the volcanic point of rage - - - and then actually turning into that monster by killing his parent during the Dharma purge.
For Sayid, it was taking the place of his older brother who could not get a grip on killing an animal for the family meal. When Sayid took control of the situation and took the animal's life so easily, it turned the young boy into a stone cold assassin and torturer because he could tune out his emotions.
For Jin, it was watching his aging fisherman father struggle to survive. He resented that he was poor and that he had no mother. He had a dream to leave poverty for the city where he would not have the daily dirty grind of trying to survive. When he was in the army, he got the taste of a bigger world than his fishing village. So he vowed to do whatever necessary never to go back there - - - and that would include compromising all his morals and beliefs.