Matthew 6:16-18 “When you fast, do not look somber as the hypocrites do, for they disfigure their faces to show others they are fasting. Truly I tell you, they have received their reward in full. But when you fast, put oil on your head and wash your face, so that it will not be obvious to others that you are fasting, but only to your Father, who is unseen; and your Father, who sees what is done in secret, will reward you.
The easy answer is “because He said when you fast”, not if. When I played sports I always wanted to exercise and run less and before practice. It wasn’t until a match in which it took 46 games before it ended that I understood why we ran so much. My partner was sick and needed extra time between games to be sick. He gave it all he had but it wasn’t what he usually could. It definitely stretched me beyond my abilities. I’d love to say we won, but we lost the third set 14-12. But when I left the court defeated people came up to me as if I had won. When I see people who watched the matched they can’t remember who beat us, but what they talk about is the effort we showed on the court. I could because my coach made me run so much.
Fasting prepares us for the battles ahead. It might not make sense to you now, but when it seems that all is lost and you find hidden within you the strength to continue you will understand. Jesus knew it and did it before anything else.