See Matthew 5:46-48.

For sinners also love those that love them - I believe the word ἁμαρτωλοι is used by St. Luke in the same sense in which τελωναι, tax-gatherers, is used by St. Matthew, Matthew 5:46, Matthew 5:47, and signifies heathens; not only men who have no religion, but men who acknowledge none. The religion of Christ not only corrects the errors and reforms the disorders of the fallen nature of man, but raises it even above itself: it brings it near to God; and, by universal love, leads it to frame its conduct according to that of the Sovereign Being. "A man should tremble who finds nothing in his life besides the external part of religion, but what may be found in the life of a Turk or a heathen." The Gospel of the grace of God purifies and renews the heart, causing it to resemble that Christ through whom the grace came. See the note on Luke 7:37.

For if ye love them which love you, what thank have ye?.... Or, "what grace have ye?" this is no fruit, nor evidence of grace, nor any exercise of the true grace of love; nor is it any favour conferred upon the object loved, which deserves the respect shown, nor can any reward be expected for such treatment: and thus it is expressed in Matthew, "what reward have ye?" and the Arabic version renders it so here:

for sinners also love those who love them: men that are destitute of the grace of God, profligate sinners, even the worst of them, such as publicans, do this; See Gill on Matthew 5:46.

What thank (ποία)?

What kind of thanks? Not what is your reward, but what is its quality ? On thanks (χάρις), see on Luke 1:30.

For if ye love them which love you, {g} what thank have ye? for sinners also love those that love them.

(g) What is there in this your work that is to be accounted of? For if you look to have reward by loving, seek those rewards which are indeed rewards: love your enemies, and so will you show to the world that you look for those rewards which come from God.

6:32 If ye love them which love you, etc. See the note on Mt 5:44.

6:32 It is greatly observable, our Lord has so little regard for one of the highest instances of natural virtue, namely, the returning love for love, that he does not account it even to deserve thanks. For even sinners, saith he, do the same: men who do not regard God at all. Therefore he may do this, who has not taken one step in Christianity.

Margin sinners

See Scofield Note: "Rom 3:23"

6:27-36 These are hard lessons to flesh and blood. But if we are thoroughly grounded in the faith of Christ's love, this will make his commands easy to us. Every one that comes to him for washing in his blood, and knows the greatness of the mercy and the love there is in him, can say, in truth and sincerity, Lord, what wilt thou have me to do? Let us then aim to be merciful, even according to the mercy of our heavenly Father to us.