Matthew 5:43-48

Loving your enemies, and the demand for perfection.
v.43-44 The OT tells us to love our neighbor (Lv 19:18). Jesus is here preaching against, not the OT, but an interpretation of the OT that said "hate your enemies." The true intent of Lv 19:18 includes loving your enemy. Lk 10:25-37 addresses the question "who is my neighbor?"
v.45 The (true) sons of the Father are those who respond to Jesus' teaching. God shows his love to all men without distinction. It rains for both the evil (unjust) and the good (just) man (but of course God does make a distinction between the evil and just, i.e., Mt 25:31-46). We are to love (and therefore pray for) our enemies. Jesus prayed for those who were persecuting him (Lk 23:34).
v.46-47 Tax collectors loved those who loved them (mother, other tax collectors?). Jesus asks what reward will you have for doing the same. Similarly, gentiles greet (only) their brothers, so what reward can you expect for doing the same. We are called to a higher standard. I take the rewards mentioned in these verses to be those you store up and have waiting for you in heaven (Mt 6:19-21).
v.48 Similar to Lv 19:2, here Jesus teaches that we are to be perfect (as God is perfect). His followers must emulate his perfection.

Read Matthew 5:43-48