Does God owe us anything? No. many are thankful because they expected what they received and thought they have earned it. We are not to be seeking great things for ourselves. Because God has been so gracious, or sacrifice of our bodies should seem perfectly reasonable. We should acknowledge God's rightful ownership of our body, we are simply stewards of our bodies. As believers, our spirit and soul are already His, but the body has remnants of the old man so we need to mortify it and therefore to offer it daily. It is a sacrifice made while living, and living daily.