I don't know the source for this, but it sums things up nicely for me.

One evening an old Cherokee 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. It is anger, envy, jealousy, sorrow, regret, greed,
arrogance, self-pity, guilt, resentment, inferiority, lies, false
pride, superiority, and ego.

The other is Good. It is joy, peace, love, hope, serenity, humility,
kindness, benevolence, empathy, generosity, truth, compassion and
faith.'

The grandson thought about it for a minute and then asked his
grandfather: 'Which wolf wins?'

The old Cherokee simply replied, 'The one you feed.'