What the Bible says about Servant always a Debtor of Service
(From Forerunner Commentary)
The only limit to the servant's duty is his master's will. There is no point at which we can claim that we have done enough and are entitled to ease. The servant is always a debtor of service; the master is never a debtor of reward. One who idolizes his duty may be satisfied when his duty is accomplished and expect the praise of others, but servants should not expect even thanks.
God promises us rewards, but we do not work for the Master simply to receive compensation. As servants, we serve Him because we are His to command as He wills and because we love Him. He has every right to our service and is under no obligation to thank us for our obedience. The servant does not serve for nothing, but receives consideration for the gift of salvation because of his dedicated obedience and humble service. Nevertheless, it is good for His servants to seek His praises and rewards with the right attitude because God does praise and reward the faithful (Colossians 3:23-24).
Martin G. Collins
Parable of the Unprofitable Servants