Great biblical principles are taught with slaves in mind. We have been purchased by the blood of Christ (Acts 20:28; 1 Pet. 1:18-19). The position of one who has been purchased is not that of doing his own will, but the will of the one who paid the price for his freedom. The benefactor is Christ. So, belong to him.
Paul said, "And that he died for all, that they which live should not henceforth live unto themselves, but unto him which died for them, and rose again" (2 Cor. 5:15). Paul also said, "I beseech you therefore, brethren, by the mercies of God that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service" (Rom. 12:1).
Christ paid the purchase price (his blood) for each one of us, we can understand why he expects us to serve him with our bodies, our souls, and our strength.