Why does Bernard of Clairvaux characterize some people as slaves, some as mercenaries, and some as sons, with regard to their relationship with God?

Why does Bernard of Clairvaux characterize some people as slaves, some as mercenaries, and some as sons, with regard to their relationship with God? 



Answer: The slaves praise God for his power, mercenaries for the benefits, and the son praises God for his goodness.