Love of Enemies

Jesus urged his followers to love other people, even those who hurt them. He said:

My command to you is: love your enemies, pray for your persecutors. This will prove that you are sons of your heavenly Father, for his sun rises on the bad and the good, he rains on the just and the unjust. If you love those who love you, what merit is there in that? Do not tax collectors do as much? And if you greet your brothers only, what is so praiseworthy about that? Do not pagans do as much? (Matthew 5:44-47)

What reasons does Jesus give here for trying to love everyone, not just friends and family?