Matthew 6:16 “And whenever you fast, do not look dismal, like the hypocrites, for they disfigure their faces so as to show others that they are fasting. Truly I tell you, they have received their reward. 17 But when you fast, put oil on your head and wash your face, 18 so that your fasting may be seen not by others but by your Father who is in secret; and your Father who sees in secret will reward you."
The Sermon on the Mount (Matthew 5-7) is at the center of all of Jesus' teachings. He unpacks a year's worth of sermons in one sermon.
He addresses fasting in the above verses...but notice where He went right before that...Matthew 6:1 Be careful of practicing your piety for others to see, for then you have no reward from your Father in Heaven.
He then talks about giving (Matthew 6:2-4) and says if you're showing off when you give resources to the Kingdom, your reward will only be that others will see you (which Jesus suggests was your motivation to begin with)
He then talks about praying (Matthew 6:5-15) and warns that if your prayer life is only geared toward the public eye, the public eye is essentially all who will hear your prayer (which Jesus suggests was your motivation to begin with)
He then goes into fasting. Again, if you are putting theatrics to your fasting, then you will receive nothing from God and only have others watch your performance (which Jesus suggests was your motivation to begin with)
This can manifest itself in lots of ways...You might announce that you are fasting everytime there is food that is not on your "To-Eat-List". It can show up as you dramatize excruciating suffering, whether headaches, hunger or some other drama. It can show up in self-righteousness as you scowl at people who are eating far less healthy than you. It can show up in your focus being solely on the weight you're shedding (or not shedding) on the scale.
Jesus is warning...Check yourself before you wreck yourself. He knows our motivations and if we are out of tune with the rhythms and melodies of His grace, He will know...and so will others around us.
But if we press in more and more to His presence...more and more into His sustaining love and grace...more and more into the cross...He will reward us (not with a juicy burger...not with a check in the mail...but with MORE OF HIM. That is the ultimate reward. So the blessing we receive when we give, when we pray and when we fast with pure and right motives is HIM.
And those who have experienced this truly and richly know there is no greater reward.