During the preparations for the burning, the entrails and legs—representing our innermost being: the heart from which conduct springs; the viscera, our emotions; and the legs, our walk—must be cleansed with water before all is burned on the fire. The burnt offering is cleaned on the inside and then completely consumed.
Here is pictured the standard of devotion to God; this is what God is aiming His children toward due to our access to Him through Christ. We are to be a cleansed, total sacrifice. We are to withhold nothing; we are to give our all. This is the hardest of all the offerings God calls upon us to perform because, like the rich young ruler, we want to reserve things for ourselves. Whatever it is, it is like a child's security blanket, and we love it and do not want to let it go.
David understood sacrificing, which II Samuel 24:24 reveals:
Then the king said to Araunah. "No, but I will surely buy it from you for a price; nor will I offer burnt offerings to the LORD my God with that which costs me nothing." So David bought the threshing floor and the oxen for fifty shekels of silver.
The burnt offering is painful because it is costly. It is so costly because it costs us our life. This is what we give in exchange for the forgiveness of our sins! Jesus Himself says this in Luke 14:26: "If anyone comes to Me and does not hate his father and mother, wife and children, brothers and sisters, yes, and his own life also, he cannot be My disciple."
Hebrews 5:7-8 informs us that Jesus Christ felt His sacrifices—not just His sacrifice on the stake, but also the multitude of sacrifices He made after emptying Himself of His godly prerogatives to live as a burnt offering for 33½ years.