The argument here is that if God is merciful and loving enough to make the sun rise and the rain come down for the good and the bad, how much more should we love the just and specially the unjust. This is a hard statement, because the nature of man (sinful) will always strive towards vengeance and "an eye for an eye and a tooth for a tooth". Let us humble ourselves and pray for those who despitefully use us and persecute us so that our fruit is that of righteousness

Matthew 5:44 and Matthew 5:45