We can get an idea of the importance of loving-kindness from the fact the Men of Great Assembly composed a special prayer of praise about it at the end of the Shemoneh Esrei. There we say: “For with the light of Your countenance You gave us, Hashem, our God, the Torah of life and a love of kindness.” The expression “the light of Your countenance” is an allusion to the event at Mount Sinai, at which the Holy One revealed several basic and fundamental matters. One such revelation was that the existence of the entire universe is dependant on the Torah, and that if there would be even one moment during which all inhabitants of the world are idle from Torah study, then all of the worlds would instantly vanish, as described in the holy Zohar.
The second fundamental lesson was that man must pursue the trait of kindness and to love it with one’s whole heart, for the whole existence of the world is all thanks to Hashem’s kindness—if Hashem were to lead the world with the trait of strict justice, the world could not exist. It is therefore proper that man, as well, grasp the trait of kindness, and in this way he gains existence in this world and the next.