Matthew 6:1 is the theme verse for Matthew 6:2-18. In fact, 6:1 is the principle and 6:2-18 contains three examples. What we find in 6:1 pertains to giving, to praying, and to fasting. The theme or the principle of 6:1 is simple, simple, simple:

Beware of practicing your piety before others in order to be seen by them; for then you have no reward from your Father in heaven.

I’ve often said this: it is not what we don’t understand about the Bible but what we do understand that should concern us. What we understand here is that our spiritual disciplines of giving, praying, and fasting are never, ever, not once to be done in order to gather attention to our spirituality. We are to leave no footprints behind us when we walk — that is, we are not to leave visible, notable evidence that “we did this or that.”

Three points:
First, we need to “beware” of the problem: there is a human need, call it an urge or a lust, to be noticed for the good we do. It all begins by recognizing that this is a human problem that must be resisted. The purpose of spiritual work is not congratulations.
Second, the summons of Jesus here is to find followers who will not do spiritual practices to be seen by others.
Third, the consequence of not heeding Jesus’ words is crassly Jesus-like: Jesus often speaks of rewards. There is simply no reward with God when we do things to be seen by others. “Reward” with God here speaks of spiritual intimacy and final approval by God — blessing, reward, call it what you will. Those who do their spiritual disciplines not to be seen are those who will find intimacy with God and reward from God.
Some focus here on altruism or purity of heart or motivation — and there is no way under heaven that you and I can ever achieve purity of heart or devoted motivation that is focused solely on God. But that doesn’t keep Jesus from telling his followers that they are to strive for it. So we should.
How so? Might I suggest that we begin by focusing on what we are doing, asking for God’s Spirit to attend, and to go at it with might and mane? There are no simple solutions, for if there were Jesus would give them.