Do nothing out of fear or a sense of duty; do everything out of a love for God.
When Jesus asked Peter whether Peter loved Jesus "more than these," Peter did not respond unreservedly but rather the Holy Spirit indicates that Peter used a different word in his response from the word which Jesus used.
Jesus asked Peter a second time but this time, Jesus dropped the phrase "more than these" from the question and again Peter responded with a change in wording.
The third time Jesus asked, Jesus used the word that Peter had been using and this time Peter became upset.
A little later, Peter said "But Lord, what about this man?"
Which shows us that Peter was still not fully surrendered in his heart.
The point of what I am saying is that Jesus accepted Peter before Peter had fully surrendered to Jesus.
Later, Paul accuses the Corinthian Christians of still being carnal-minded. Their minds were not fully transformed, therefore their hearts did not possess the degree of love that a Christian should seek.
Just as parents, do not expect a newborn to hop, skip, jump, walk, and run—God does not expect full grown maturity as soon as a person accepts God's summons.
It is true that a Christian should aspire to eventually having a heart which does nothing out of fear or duty—but there is a reason that the Bible condemns those who lack a fear of God. The grace of God accepts the fearful and the immature who are still "doing their duty" as babes who still need to grow in the grace and knowledge of Jesus.