Consider two types of servants. Both devotedly carry out the master’s instructions, and enjoy doing so, but their motivations differ.
The first servant prides himself at his accomplishments. Human nature is such that a person values and feels fulfilled from something in which he invested personal effort, even if the product of that effort is of objectively poor quality. In contrast, the accomplishments of others bring him no sense of fulfillment, even if those accomplishments are qualitatively far superior. It is this type of pleasure that motivates this servant to obey. Although this servant carefully adheres to his master’s instructions, he is essentially submitting to the master’s wishes, and not to the master himself.
The second servant feels a profound inner bond with his master. His deepest yearning is to bring satisfaction to his master, and his greatest source of personal fulfillment is the knowledge that he has succeeded in doing so. The more he comes to realize his master’s greatness, the more elated he is at his merit to bring the master pleasure.
Similarly, there are two different levels of devotion to G–d:
The first person follows Jewish law carefully and prides himself on his observance, but this pride stems from his comprehension of the meaning of the Mitzvos, and a concomitant sense of accomplishment at his observance. He relates better to the Mitzvos than to their Commander.
The second person has attained a truly personal relationship with G–d. He loves G–d from the depth of his soul, and yearns desperately to please Him, and this desire motivates his observance of Mitzvos. The more he studies G–d’s greatness, the more intense and deeply felt is his aspiration to bring pleasure to G–d.
One way of discerning one’s level is by asking oneself whether he differentiates between the Chukim, the suprarational Mitzvos (such as Sha’atnez) and the Mishpatim, the rational Mitzvos (such as the Mitzvos designed to promote peaceful interactions between people).
If he performs the Mishpatim enthusiastically and the Chukim sluggishly, this demonstrates that his main motivation in his observance of Mitzvos is to attain personal satisfaction.
However, if he is excited at the opportunity to perform any Mitzvah, even those which he does not comprehend at all, simply because he yearns to perform the will of G–d, then he knows that he is devoted to G–d Himself, and not merely to His Mitzvos.
Adapted from the Frierdiker Rebbe’s Reshimas Chag HaShavuos 5675, with references and explanatory notes from the Rebbe.
 Bava Metzia 38a.
 The prohibition of wearing clothes that contain a mixture of wool and linen.
 In the original, this is the difference between the bittul to the ratzon and the bittul to the ba’al ho’ratzon.