Saw an interesting article Study: People ‘Invest in Karma’ When Facing Uncontrollable Events where a group of researchers have conducted experiments with on same.
People who tend to help us we help them first. That is a natural rule. Even animals (most importantly) remember very well people who fed them food and looked after them in their adverse times. They tend to show reciprocity. So, there is a natural need to accumulate Good karma. It is an inherent quality.
Which means if we do not help others we will not be helped as well. I guess this quality we acquire from within our genes.