Matthew 6:17 -18 But you in fasting, anoint your head and wash your face, so as not to appear to men to be fasting, but to your Father in secret. And your Father seeing in secret will repay you in the open.
Another way of saying the above text is, “When you fast, do as you always do. Don't change the way you look or act to let others know about your fasting.”
Fasting is something that is between the person and God. When a person draws attention to their fasting they are drawing attention to themselves and away from God. It also makes the fasting a meaningless act, done only to draw the attention of others.
The Jews, and the nations around them were in a habit of washing and anointing their bodies. The washing was done at every meal and their bodies were daily anointed with sweet, or olive, oil. In a climate as warm and dry as was theirs, this anointing kept the skin smooth, healthy, and perfumed. When they fasted, they would stop doing the washing and anointing. In the Jewish canon it was forbidden to wash and anoint on days of fasting and humiliation and some people would use this law to make it appear they were fasting and draw attention to themselves.
God wants our worship to Him to be genuine. He wants our love for Him to be genuine. He wants the real you and the real me. That is why He asks us to keep our fasting private, to pray in private (Matthew 6:6), and keep our charitable deeds in private (Matthew 6:1-4). That is not to say all prayer and worship need to be in private. Prayer and worship in church is very public. Prayer and worship with small gathering of friends is public. But God also wants to hear from us in private. How a person behaves in private is usually a true reflection of who they really are. Whenever there are other people around, the temptation to look good, to impress others, is always present. The only way God can have our true, genuine, undistracted, worship and love, is to ask us to come to Him in private. It is also the only way we can give Him our full attention so He can communicate to us. Fellowship with God, sensing His presence, being comforted and encouraged by Him, is our reward. There is nothing more rewarding than being in the presence of God and having the peace of mind which comes from spending time alone with Him.