But love your enemies, do good to them, and lend to them without expecting to get anything back. Then your reward will be great, and you will be children of the Most High, because he is kind to the ungrateful and wicked. Be merciful, just as your Father is merciful.-Luke 6:35-36
The parallel passage for this can be found in Matthew 5 with says:
You have heard that it was said, “Love your neighbor and hate your enemy.” But I tell you, love your enemies and pray for those who persecute you, that you may be children of you Father in heaven. He causes his sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous. If you love those who love you, what reward will you get? Are not even the tax collectors doing that? And if you greet only your own people, what are you doing more than others? Do not even pagans do that? Be perfect, therefore, as your Heavenly Father is perfect.-Matthew 5:43-48 NIV
Both passages are teaching us the exact same thing, and that is to love everyone we come into contact with. And we are to do this without expecting anything in return. And why are we to do this? First of all because God has commanded us to do it, but more importantly because it is how God loves us.
Before Paul begins has great discourse on the importance of love of says this: “And yet I will you show you the most excellent way” (1 Corinthians 12:31 NIV). And that most excellent way is love. And love is what is enables you to be merciful. Because as Paul said:
Love is patient, love is kind. It doe not envy, it does not boast, it is not proud. It does not dishonor others, it is not self-seeking, it is not easily angered, it keeps no record of wrongs. Love does not delight in evil but rejoices with the truth. It always protects, always trusts, always hopes, always perseveres.-1 Corinthians 13:4-7 NIV