A Native American elder once described his own inner struggles in this manner: Inside of me there are two dogs. One of the dogs is mean and evil. The other dog is good. The mean dog fights the good dog all the time. When asked which dog wins, he reflected for a moment and replied, The one I feed the most.
Georges Bernard Shaw
Assuming that the 2 sides , good and bad are existing in human, Shaw considered that each side will grow based on how and what we feed our mind! Many other conditions can affect that behavior, such as friends we frequently meet, environment and circumstances which can push either way.
We have seen people who were extremely good transformed into bad side, based on incidents which happened to them and affected them negatively, and the opposite is correct!
That quote remind us that if we have the control over the choice, to always strive for the goodness and avoid falling into the bad side.