The magisterial Reformer John Calvin, explains the reason for God’s compassion towards His chosen people, while preaching through the book of Malachi as follows:
As we have said, there is no real difference among men, except in their hidden election. Some theologians would make foreknowledge the mother of election, and that very foolishly and childishly. They say that some men are chosen and others rejected by God, because God, from whom nothing is hidden, foresees of what sort each man will be. But I ask, Whence comes virtue to one and vice to the other? If they say, “From free will,” surely creation was before free will. This is one point. Besides, we know that all men were created alike in the person of Adam. . . . And what does this mean except that the condition of all who come from the one root is the same?
I am not discussing “special gifts.” I admit that if our nature had not been corrupted and we all had the same assurance of blessedness, we would be endowed with a variety of gifts. . . . But since in Adam all are sinners, deserving of eternal death, it is obvious that nothing but sin will be found in men. Therefore, God’s foreknowledge cannot be the reason of our election, because when God [looks into the future and] surveys all mankind, he will find them all, from the first to the last, under the same curse. So we see how foolishly triflers prattle when they ascribe to mere naked foreknowledge what must be founded on God’s good pleasure. . . .
When Moses prays to God not to break his covenant with Abraham, God answers, “I will have compassion on whom I will have compassion.” What does he mean? He means that the reason for God’s keeping some for himself and rejecting others is to be sought nowhere but in God himself. When he says, “I will have compassion on whom I will have compassion,” the repetition may seem empty and dull; but it is in reality emphatic. . . . The reason for compassion is compassion itself. 
 John Calvin, Calvin: Commentaries, The Library Of Christian Classics, Volume XXIII, (translated and edited by Joseph Haroutunian, with Louise Pettibone Smith), The Westminster Press, p 294-295