-Matt. 6:4 (NLT)
The Christian life is a life lived in Christ. He gives us a new life that we live through him, all week, 24 hours a day. The gifts we give, the good deeds we do, and the generosity that we live in - all need to be done in secret, privately, off-stage, and outside the spotlight of the attention of others.
If you do your good deeds, your giving, or your acts of righteousness when no one is looking, but God; then God will reward you. God will return the favor. God will pay you back. God will reimburse you.
If you do good deeds on stage, in the spotlight, and in any way that is in the open, saying, "hey, look what I am doing!", then the attention and admiration you get is your payment. And, you get nothing from God. If you do good, only in front of others, showing off, with a desire to been seen; Jesus says, "you have your reward."
There is nothing wrong with being seen doing good and we often can not help being seen, but the point is that if you are doing it purposely in front of others, publicly, or 'on stage'; then your reward is from your audience and not from God.
I believe that Jesus knows that we love affirmation and we do want to look good and be seen as people who do good things. But, he is warning us that when you represent God, that you have to be very careful not to do good for the affirmation and applause of the people.
Something ugly in us grows as a result of doing good, so that we look good. We need to be people that do good, because it is the right thing to do, directed by the goodness and the righteousness of God.
The ugliness that we don't want is to be a person that does good, but is only motivated by looking good and not by God's righteousness and goodness. That ugliness is called self-righteousness. Where that road leads is that we treat people as extensions of ourselves, becoming users; we draw people to ourselves, instead of to God or Christ; and we build monuments to ourselves.
God is real and authentic. We, who are Christians and disciples are humans, who are in a transformational process where we are trading our selfish life for the Lord Jesus' life. We are dying daily and working out what he works in. We are in a process, saying, "be patient with me, God is not finished with me yet".
If you, as a human Christian, are not in that process; then your carnal, un-dead, self-life will be clashing with the life of Christ, on the playing field of your religion. In your carnality, you will want to show off and get attention and be self-righteous. It is just natural for humans to do that, outside of Christ.
On the other hand, the disciple pays attention to Jesus' words, when he says, "watch out!", or "be careful!" He says, "don't fall into the hypocrisy trap, where you do one thing on stage and another thing in private." He also says to not call attention to your self or show off when you do good. Because you will grow in the ugly and not have a reward from God.
Don't you want God's rewards? Jesus is saying that the religious people of his day, preferred the applause of people rather than God's applause. This is something so strange, because these people would talk about God all the time, but they were not serving God, but serving themselves. We do not want to be like that.