The simple fact is, God never told us why He made a difference between "clean" and "unclean" animals when Noah loaded the ark. One reason may be because people in the Old Testament were commanded to offer the best of their clean animals as sacrifices to God. Perhaps by making certain animals "clean" and others "unclean," and then requiring that only the clean animals could be used as sacrifices, God was able to teach people that He always deserved their very best. When one of King David’s men offered him an ox free of charge to use as a sacrifice to God, the king said: "Nor will I offer burnt offerings to the Lord my God with that which costs me nothing" (2 Samuel 24:24). David knew that God deserved better. Although we cannot be certain, it seems that God first began making a difference between "clean" and "unclean" animals so that people would know that He wanted their best.