Jesus commanded you to love (Mark 12:31). Paul said that if you do not have love, you are nothing (1 Cor 13:2). And John said “We know that we have passed from death to life, because we love the brethren. He who does not love his brother abides in death” (1 John 3:14).
Love is key, but what does it mean to love?
Think on this quote from Martin Luther, showing that the greatest commandment is both simple and complex:
This commandment of love is a short commandment and a long commandment; it is one single commandment and many commandments; it is no commandment at all and it is all commandments. That is, it is short and one in itself, and as far as the intellect is concerned, one grasps it quickly. But it is long and many from the point of view of practice, because it encompasses all commandments, being the master commandment. Furthermore, it is no commandment at all if one thinks about works, because there is no one single particular work related to it. However, it is all commandments, because the works of all commandments are, and must be, works of this commandment. Thus, the commandment of love annihilates all commandments and yet establishes them all (Quoted in Mannermaa, Two Kinds of Love, p. 68).