As a child I remember running up to my mother on multiple occasions and shouting the phrase "hey mom, watch this!" Before I did anything that I deemed "impressive" I would always try to get the attention of my mother or father so that I could make sure that they were prepared to bask in the glow of my soon to be "impressive" action. I was not content with just doing the action, I wanted to be recognized for it as well.
We see this same kind of mentality in Matthew 6. In Matthew chapter 6 Jesus is going to discuss the problem with "practicing your righteousness" before men (6:1). Those people who do these righteous acts before men are referred to as "hypocrites" (6:2, 5, 16). Jesus goes on to discuss three areas where these hypocrites (Pharisees in this context; 5:20) were practicing their righteousness before men. These three areas are giving (6:2-4), prayer (6:5-15) and fasting (6:16-18). The Pharisees took these righteous acts and turn them into something twisted. How? They sought to be rewarded by men. They were acting just like I was when I was a child.
Unfortunately, this same kind of mentality is around in our minds today. While we may not like to admit it, many of us do the right things, for the wrong reasons. Maybe we do some good deed and then write it up on Facebook so everyone can hear about it. Maybe we hold up our wad of cash so it can be seen by everyone before we put it in the collection plate. Maybe we talk to our friends about just how hungry you are and how hard this fasting thing is. Maybe you get up to pray and you use the biggest, fanciest words you can think of.
Doing these kinds of things is essentially like shouting "hey everybody, watch this!" If we can learn anything from this section of Scripture it's that when we do actions to receive the glory from man, we get no reward from God (6:3-4, 5-6, 16-18). The question then for us: who is watching you? Are you doing good so that men will see you, or so that God will see you? Do you want your reward from men, or from God? With men the reward is minimal and fleeting, but with God you have a reward that will not be destroyed (6:19-20).