Toras Kohanim cites Rabbi Akiva who says that this is the fundamental rule of the Torah. The Mizrachi cites the Talmud in Shabbos 31a in which Hillel says, “What is hateful to you, do not do unto others.” This is what Hillel told the gentile who wanted to convert to Judaism. If the Torah conveys this mitzvah in a positive light, encouraging us to love our fellow, why do Chazal seek an interpretation that emphasizes and focuses on the negative? Horav Simchah Scheps, z.l., cites Horav Yerucham Levovitz, z.l., who explains the Mishnah in Pirkei Avos 3:2 in the following manner:
Chazal teach us to “pray for the welfare of the government, because if people did not fear it, a person would swallow his fellow alive.” Rav Yerucham wonders why this Mishnah is placed in Meseches Avos, which deals with ethics and interpersonal relationships with people. He explains that Chazal are teaching us a compelling lesson. If not for fear of reprisal from the government, man is capable of descending to the nadir of depravity to overwhelm and subdue another person who might stand in his way. He is capable of swallowing him up! Veritably, we have only to peruse world history, or even to glance around at those uncivilized countries in which anarchy reigns and fear of reprisal is something of the past, in which murder and plunder are a way of life.
With this idea in mind, Rav Sheps explains why Chazal chose a negative approach towards explaining a positive commandment. It is essential that we understand that the only way we are able to control the forces of evil within us is by understanding with absolute clarity that what we do not want for ourselves, we should not do to our fellow. This means that one can achieve ethical behavior and form humanistic relationships only through Torah study and mitzvah observance. It is impossible to develop true ethical character without Torah. One cannot hope to observe the golden rule, “Love your fellow as yourself,” unless he realizes that he must first eradicate his negative attitude towards others. This can only occur with the support and guidance of the Torah. Ahavas Yisrael is the natural consequence of an acute understanding that one may not do to others what he does not want done to him. This can only be achieved through the vehicle of Torah.