Be doers of the word is a repeating theme of James. In previous verses James emphasized on hearing and told us that we are to hear the words!! And this week he goes one step further. If we hear the words, we now should do what we hear. It means we should follow the words and have the change. And these changes can be either our action or our knowledge and emotion.
The reason is that making change is the purpose of hearing. If we don't do, then there's no point of hearing. Also, God's words are always true. It's because either God speaks truth, or what God speak becomes true. Therefore, following the words of God is important. If we don't do, we have nothing to do with God.
And James explains this deceit as a mirror. At that time, the mirror was dim, and people saw their image quickly and go away. It is as same as merely listening the words. We listen and do not stay in the words but go our own way. This is not doing but deceiving.
However, those who looks into the perfect law and perseveres, are the ones who keep thinking the words and live out. In Matthew 21, Jesus gives a story of two sons. When their father told them to go to work in vineyard, one said yes and didn't go, while the other said no and later changed his mind and go. Jesus asked a question, 'which of two did the will of the father?' And I will add one more question, 'which of two deceived the father?'