This story just about sums up my beliefs on the topic:
One evening an old man told his grandson about a battle that goes on inside people. He said, "My son, the battle is between two wolves inside us all."
"One is evil - he is anger, envy, sorrow, regret, greed, arrogance, self-pity, guilt, resentment, inferiority, lies, false pride, superiority and ego."
He continued, "The other is good - he is joy, peace, love, hope, serenity, humility, kindness, benevolence, empathy, generosity, truth, compassion and faith. The same fight is going on inside you - and inside every other person, too."
The grandson thought about it for a minute and then asked his grandfather, "Which wolf will win?"
The old man simply replied, "The one you feed."