Our morning prayers are prefaced by the following line: "It is proper to say before prayer: I hereby take upon myself to fulfill the mitzva, ‘Love your fellowman as yourself.'"
When a Jew prays, he is symbolically offering his soul before G-d. In the same way that a physical sacrifice had to be whole and unblemished, so too must a Jew be without defect before approaching his Maker. The Jewish people are considered to be a single body; if a Jew does not properly love one of the "limbs," the entire body is affected and becomes flawed. It is therefore appropriate to accept the commandment to love our fellow Jews before prayer to ensure that the "sacrifice" we offer is perfect and whole. (Ohr HaTorah of the Tzemach Tzedek)