Good question. I think that this is sadly not true all of the time. I wish it was. But my short answer is that most of the time, what goes around does come around. I think this not because of any spiritual slant but because when we lead our lives in a certain way we will tend to have similar consequences. When we strive to help others, we tend to place ourselves in environments that will give back. When we are complacent to, or active in, harming others then we tend to lead a life in this vein and will get similar actions done unto us. Hope that all made sense.