It might be difficult for us nowadays to speak to servants or employees the way people spoke to servants (mostly slaves) in Jesus' time. But the point that Jesus is making is an important one. We can never really do anything to 'earn' the love of God. We cannot manipulate God. God is never in debt to us. Everything we have comes from Him. Even the love we show to Him and our brothers and sisters comes from Him. We cannot give to God what he does not already have. We can only give back what he has already given to us. We cannot act like the Pharisee in the Temple who despises the tax collector praying behind him. The Pharisee more or less implies that God should be grateful to have someone who keeps the Law so well. When we have done everything we should do we can only say: "We are merely servants; we have not done more than our duty". Or as one of the weekday prefaces of the Mass puts it: "You have no need of our praise, yet our very desire to thank you is itself your gift." Service as understood by the Gospel is not in any way to demean or lower oneself. Rather it is to act in a loving and caring way for all those around us. That is our duty, and that is where our happiness lies.