Dealing with Matthew 6:16-18, The Expositor’s Bible Commentary has the following to say about fasting:
Hypocrisy is not the sole preserve of Pharisees. The solution is not to abolish fasting (cf. Alexander’s remark that mortification of the flesh “can be better attained by habitual temperance than by occasional abstinence”) but to set it within a biblical framework (references on v.16) and sincerely to covet God’s blessing. For if the form of vv. 1-18 is negative, the point is positive – viz., to seek first God’s kingdom and righteousness (cf v.33).
The Expositor’s Bible Commentary, p.176