Serving Hashem Without Expectation of Reward
The third mishnah in the first perek of pirkei avos discusses the optimal attitude one must assume to achieve true yiras Hashem (fear of G-d):
Al tiheyu k'avadim ha'meshamshim es harav al menas l'kabel pras; elah heyu kavadim ha'meshahshim es harav shelo al menas l'kabel, v'ihi morah shamayim aleichem
Be not like servants who serve the master on condition of receiving reward. Rather, be as servants who serve the master without the condition of receiving reward, and fear of Heaven will be upon you.
What seems unclear is why being motivated by reward (and deterred by punishment) should detract from one's yiras Hashem. If one tries to do the right thing (and avoid the wrong things), motivated by a desire to obtain reward (and avoid punishment), why should that person's yiras hashem be less? Indeed, isn't fear of punishment synonymous with yiras Hashem? I try to avoid sin in order to avoid Hashem's wrath, and pursue good in order to obtain Hashem's blessing. What is wrong with that?
Yet, in the fourth chapter of Mesilas Yesharim, we see that reward and punishment is considered the lowest form of motivation for choosing good and avoiding evil (which R' Luzzato refers to as "vigilance" or zehirus). The highest motivation for exercising vigilance in one's actions is the realization that perfection of character is an ideal in and of itself, irrespective of reward.
Insights provided by Rabbi Dovid Orlofsky of Jersualem in a recently taped Tisha B'Av shiur can help shed some light on these issues. Briefly, R' Orlofsky described 3 types of relationships with Hashem by analogy to relationships with other people with which we are all familiar in our daily lives. Since we understand the dynamics of these human relationships so well, the analogy can help us model a relationship with Hashem of the kind described in the mishnah. That is, because we understand which relationships in life give us the most satisfaction and pleasure, we can begin to understand why a relationship with Hashem that is not based on reward will be the most spiritually uplifting.