There are two general modes, or levels, of serving God, two realms of connection between a human being and God.
The first is that of a servant, as in, “For to Me the children of Israel are slaves” (Lev. 25:55).
This means that a person serves God even without a reason and without understanding, like a servant, who does not need to know the reason for his master s commands but obeys them without a reason, even when he has no intrinsic desire to do so.
The second mode is that of a son, as in “My firstborn son is Israel” (Ex 4:22).
This means that a person fulfills the commandments out of love.
The intention behind his act is to do the will of his father, whom he loves.
This does not mean that the two modes must exist separately, that some people are “servants” while others are “children.”
Rather, as stated in the liturgical hymn sung on Rosh HaShana, “whether as children or as servants,” meaning that at certain times and in certain manifestations of our being, we are like children, and at other times, and in certain manifestations of our being, we are like slaves.
Everyone should serve God with both of these modes and levels; a person should not be satisfied with only one of them.
When a person worships God as a “servant” only, there is something lacking in his intent and spiritual attachment.
However, when a person worships God as a “son” only, solely out of love and desire, something there is lacking as well.
The “son” who acts out of love is really doing so for his own gratification, whereas the “servant,” who acts out of simple acceptance of a yoke imposed upon him, does so for the sake of his Master.
A person who lacks this aspect of servitude is missing something from his divine service.
There is a flaw in the meaning and objective value of this service.
Divine service must go beyond an individual’s personal, selfish desires.
Rabbi Adin Steinsaltz