If we were to take a poll of how people describe "yiras shamayim" (fear of Heaven) we would probably receive various different answers. Some would claim that, "Yirah is being meticulous in the performance of mitzvos." Others would opine that, "Yirah is the performance of mitzvos with the proper intentions," or maybe, "Yirah is the fear of Gehinom." Rav Wolbe writes (Alei Shur pg. 500) that although each of these answers touch on some aspect of yirah, however, none have truly revealed the depth of this concept. Rashi in this week's parsha enlightens us with a deeper understanding of yirah.
"Do not curse a deaf man and before a blind man do not place a stumbling block, and you shall fear your G-d - I am Hashem" (Vayikra 19, 14). Rashi explains that the pasuk is referring to a figurative stumbling block. Do not give bad advice (which is to your own benefit) to someone who is "blind" i.e. unsuspecting with regard to the issue at hand. Because it is only the one who is offering the advice who knows his true intentions, therefore, the Torah writes, "And you shall fear your G-d" - for He can distinguish the real intent behind one's actions.
Similarly, further on in the parsha, the Torah commands us, "Rise before the aged and give respect to the elderly and you shall fear your G-d - I am Hashem" (Vayikra 19, 32). Rashi explains that a person might think that no one will know if he chooses to simply close his eyes and pretend not to see the elderly man, thereby obviating the need to stand up. Therefore, the Torah writes, "And you shall fear your G-d" because Hashem knows the true basis for one's actions.
Rav Wolbe points out that there are three more places in Parshas Behar that the Torah writes in conjunction with specific mitzvos, "And you shall fear your G-d." Despite the fact that Rashi was careful with every word he wrote, nevertheless, each and every time he explains that because it is something that only the person himself can discern - the reason that lies behind his action - therefore the Torah felt the need to write, "And you shall fear your G-d."
With this in mind, we can gain a deeper understanding of the concept of yirah. Yirah can be found in the recesses of the mind and in the rationale behind one's actions. The gauge to measure one's yiras shamayim is specifically those mitzvos that no one will know about except He Who is in the heavens. Moreover, most amazingly, all five of the above mitzvos are between man and his fellow man. It is possible to give a pious impression to the world, while deep down one has ulterior motives tainted by his personal desires. These mitzvos are the true test to see if one feels yiras shamayim in his interpersonal relationships.
The Mashgiach writes that such thoughts are possible even with regard to mitzvos between man and Hashem. One might buy a beautiful esrog so others will think that he is meticulous in his performance of mitzvos, while he might spend substantially lower than he could afford when it comes to the performance of mitzvos that no one will know about. In truth, mitzvos must be performed objectively without taking into consideration one's personal interests or other people's favorable comments.