If you lack zeal for the lost, it may be that you have never seen that your own sin makes you worthy of death and damnation. We have been justly hung over the valley of the shadow of death, with no hope outside of the mercy of God. This is why it is so beneficial to go through the Ten Commandments. Study their spiritual nature (see Matt. 5:1), and then plead with God for an understanding of what you look like to Him in light of that Law.
2 Kings 10:16 Then he said, “Come with me, and see my zeal for the Lord.” So they had him ride in his chariot.