Now, the verse "And you shall serve G‑d your L‑rd" refers to the concept of prayer,1 as Rambam states:2

It is a positive mitzvah to pray…, as it is written, "And you shall serve G‑d your L‑rd." According to the oral tradition, we learned that this teaches us that this service is prayer, as it is written:3 "And serve Him with all your heart," and our Sages state:4 "What is the service of the heart? This is prayer."5

It is understood that included in the charge to serve G‑d — in addition to the service of awe — is also the service of love which is actualized in prayer, as it is taught:6 "There is no service like the service of love."

To explain:7 Prayer produces an effect both on the G‑dly soul and the animal soul. The Hebrew term for prayer, tefillah, relatesto the term tofal which has the meaning of "joining together."8 [Through prayer,] the G‑dly soul is joined to its root and source.9 Also, [through prayer,] the animal soul is purified and refined,10 and the sparks of G‑dliness invested in one's body and one's portion of the world are elevated.11

Both of these effects are produced through the service of love. The service of fear and kabbalas ol does not change a person and it does not bring about a refinement and a purification of the animal soul. This can be observed in actual life. There are people who have natural yiras Shamayim (fear of Heaven). With minimal meditation, they are able to arouse a level of fear that motivates them to "turn away from evil and do good." Nevertheless, even when their conduct is ideal in terms of "turn[ing] away from evil and do[ing] good," their animal souls retain their inherent strength and power.12 On the contrary, their animal souls grow stronger over the course of time as they are drawn on repeatedly.13, 14

Moreover, since service motivated by fear does not bring about a refinement and a purification of the animal soul, it also does not bring about a connection between the G‑dly soul and its root and source. For the establishment of such a connection depends on the fulfillment of G‑d's will, and G‑d's will is that the soul will refine the body and the animal soul.15 That is the purpose for which the soul descended to this material plane. For the soul itself is not in need of correction.16, 17 It descends to this earthly plane only to refine the body and animal soul. As long as it has not brought about such a refinement, it has not fulfilled G‑d's will.18 Hence, it will not have connected the G‑dly soul to its root and source.

Summary

Divine service involves not only fear and kabbalas ol but also love, which is achieved through the service of prayer.

The Hebrew term for prayer, tefillah, relatesto the term tofal which has the meaning of "joining together." Prayer joins the G‑dly soul to its root and source. Prayer also refines a person's animal soul.

Both of these effects are produced through the service of love. In contrast, the service of fear and kabbalas ol does not radically change a person's inner character traits. He remains who he is; he just controls his conduct. To bring about an inner change, love is necessary. Similarly, fear does not bring about a connection between the G‑dly soul and its root and source, for the establishment of such a connection depends on the fulfillment of G‑d's will, and G‑d's will is that the soul will refine the body and the animal soul.