By asking “which commandment in the law is the greatest,” the Pharisees reveal an attitude toward law far different from that of Jesus. Instead of limiting the demand of the law as the Pharisees do to discrete commandments that are kept or not, Jesus teaches that the demand of the law embraces the totality of our relationships with God, self, and neighbour. Love defines our relationships; love is the wellspring of obedience to any commandment. Love is the greatest commandment because it truly is the whole Law of God.
….To love God with all one’s heart is to be totally attached to God. To love neighbour as self is to be as totally attached to people in one’s neighbourhood or immediate circle of friends (i.e., fellow Israelites) as one is to one’s family group. This has been and continues to be the normal way of life in the Mediterranean world, unless feuding develops.
To “hate one’s father, mother,” and others as Luke’s Jesus (Lk 14:26) requires of his followers means to detach oneself from family and join the Jesus group. Paul says the greatest among the virtues faith, hope, and charity is charity, that is, love or attachment to the group. John J. Pilch