But why does God not grant to His saints continuous overflowing joy? Since the Lord had magnified Abraham's name among the heathen, he was in danger of becoming guilty of vainglory, for our nature cannot well bear the grace of being honored and of receiving divine benefactions. God therefore for a while turns His countenance away from us and permits us to face great fears and worries. Older people are tempted less by the sins against the second table, such as theft, adultery and murder, than are younger folk. But they are threatened by far greater dangers, namely, by the sins against the first table. They are tempted to trust in their own power and to glory in their own righteousness and wisdom. With these monstrous transgressions the saints must battle throughout their life. by nature we cannot do otherwise than pride ourselves on the gifts which God grants to us, and parade them before the public. Again, when God's blessings are withheld from us, we are inclined to despair. This vicious virus I soon discovered when I studied the various stories of Holy Scripture. Therefore at the beginning of my Gospel ministry, into which God led me in His wonderful way and, so to speak, against my will, I asked Him very earnestly to deliver me from this evil and keep me from so great a sin. God indeed hear my prayer and kept me from this vice, though not to such a degree that I did not feel it. But He burdened me with so many troublesome tasks, worries, dangers and heartaches that I could easily put all vainglory out of my heart."
Martin Luther, Commentary on Genesis 15:1 (Luther's Commentary on Genesis [trans. J. Theodore Mueller; Grand Rapids: Zondervan, 1958], 1.260)