Childhood is a time when we form many of our habits that we're going to hold over later. When one has multiple negative experiences that are centered around sports early on, this can often translate into adulthood with decreased involvement (in exercise).

When you think about it, it makes intuitive sense, when you consider the hallmark signs of depression - sadness, fatigue, lack of interest in things you used to like. When kids are having a tough time with peers, and struggling with depression, then this can translate to reduced rates of physical activity.