Be perfect, just as your heavenly Father is perfect. Jesus “directive” to be perfect in imitation of God seems a rather tall order and an impossibility. Was it not Jesus himself who said that "no one is good but God alone" (cf. Luke 18:19)? Isn't being perfect the "perfection" of being  good? Was Jesus simply setting up the disciples for a failure? We pick up from where we left yesterday. Jesus continues with his teaching as he invites his listeners to join him in advancing the kingdom of God. It is an invitation that challenges them to think outside the box. 
You have heard that it was said...but I say to you. Those who had gathered to listen to Jesus were already steeped in a tradition that had come to define their identity. They had become used to a certain way of doing things. Consequently, asking them to do otherwise could easily become equated to asking them to re-define who they were. However, this is not what Jesus was asking them to do. He was simply asking them to re-evaluate their understanding of their identity. He was not suggesting to them that the laws and regulations that had guided them since the time of their fathers was a mere sham. He was not saying that there is something wrong in loving one’s neighbor and friend (reciprocating love). Loving is a commendable act, and only those who are good can love. 
Jesus knows that we are good (we are good because we come from God who is good), and that we are capable of loving those who love us. However, he wants us to do more. He does not ask us to be good but rather to be  perfect.In other words, Jesus is calling us to "better" the good that is already in us. He is not asking us to stop loving our neighbors. He is not asking us to stop reciprocating the good that has been shown to us. Rather, he is requesting that in addition to greeting those who have greeted us, we should extend our hands to those who have not greeted us. He is suggesting that instead of using too much energy to hate those who don’t love us, why not simply love them! This is what a disciple does. This is what sets a believer apart from others: going beyond the call of duty to do more. Do we have to do this? Yes, we have to, if we believe that we are sons and daughters of God who  makes the rain to fall on the just as well as upon the unjust. This is the only way to change the world. This is the only way to rid the world of hatred and the vicious circle of retributive violence. This is the only way to not only make disciples of all nations (cf. Matthew 28:19), but also to teach them how to observe the commandments of Jesus (cf. Matthew 28:20). Perfection, then, consists in refusing to hate. Perfection consists in loving beyond limits. This is why God is perfect.