These two verses clearly establish the most basic element of why we must worship God: because He commands it! He must command us to worship Him because it is possible to worship others and things besides God. Satan was clearly attempting to get Christ to worship him—a being besides God—and to that Jesus replies, referring to the Father, "Him only you shall serve." Not only does He command us to worship Him, He also forbids us to worship any others. In addition, Jesus' statement shows the inextricable link between the worship and the service of God. It is as if they are synonymous. Worship involves highly regarding and then serving the One worshipped.
By definition, we worship what we choose to give the supreme devotion of our feelings, time, and energies to. God must command us to worship Him because we can choose to give our feelings, time, and energies to things other than God. Therefore, acceptable worship of God involves consciously choosing to worship and serve only Him even in the face of the temptation to give these things to others. Notice how Psalm 45:10-11 shows that we must choose between alternatives that will present themselves from time to time. "Listen, O daughter, consider and incline your ear; forget your own people also, and your father's house; so the King will greatly desire your beauty; because He is your Lord, worship Him."
The first four commandments directly address worship. Worship refers to the supreme honor and veneration given in thought and deed to the Creator. Those four commandments plus the tenth directly influence what we are to do or not do to fulfill the minimum requirements of this duty. If we do not do so, we are guilty of idolatry. No other sin has such a direct and concentrated focus of attention. The basic requirement is that we are to worship Him alone, and to allow any person or thing to usurp that position of lordship over us constitutes gross disobedience. The first and most basic reason why we worship God is that He commands it and forbids the worship of others.