One of the wonderful things about King Benjamin is that he does not require anyone to do anything that he has not been willing to do himself. Earlier in the chapter, he told the people that he had served them, with all his "might, mind and strength" (2:11). The scriptural exhortation to love God with all of your might, mind, and strength is in several biblical passages that King Benjamin, who knew the scriptures well, would have known (see, for example, Deuteronomy 6:5; 30:6; Proverbs 2:2). He brilliantly transforms that requirement to say, in essence, "I know that I am commanded to love God with all my heart, might and mind, but, you know, I have loved you and served you with all my heart, might and mind." He is telling them, "I am your king and God has put me in as king, but I love you as much as I have loved God." And thus, he will turn around and tell them that the same opportunity and reality applies to them as well: "And when you are in the service of your fellow beings you are only in the service of your God" (2:17).
And thus, "if I, whom ye call your king … do merit any thanks from you, O how you ought to thank your heavenly King!" (2:19), because I have only been in the service of God, and he has "kept and preserved" us from day to day, lending us breath, that we might live (2:21). And thus all thankful praise should be directed toward Him. This is not only logically coherent, but also ethically right and spiritually fulfilling.
Book of Mormon Central, "How Are Acts of Service Related to Wisdom? (Mosiah 2:17)," KnoWhy 308 (May 3, 2017).