Jesus had great insight into the depths of man’s motives.  For that reason he warns us not to be like the hypocrites who try to act out when they fast.  They do this for the attention that they get from their audience.  They do this so that their actions will be validated by others.  For this reason Jesus tells us that they already received their reward, which was human attention. 

But when we fast we must not put on a show for others.  We should not even let others know we are fasting; in other words, carry on business as usual.  By fasting in this manner we are fasting for the right reasons and will be rewarded from God. 

Why do we fast? Perhaps hunger reminds us of our human frailty and dependence upon God.  Or maybe there are times that we want God to know how serious we are about our devotion to him above physical comfort.  Whatever the reason we fast it should be kept between us and God and if we do this we will receive payment. The word used here is μισθός and gives us the idea of a payment for services rather than a reward for having done something courageous.

It would be good for many evangelists to note that Jesus warned against putting on a show for crowd.  He called such people hypocrites, meaning actors.  I guess that we can assume that they also have already received their payment in the physical wealth and popularity that they have accumulated.