In response to the question about the first of the commandments, Jesus says: "The first is, 'Hear, O Israel: the Lord our God, the Lord is one; and you shall love the Lord your God with all your heart, and with all your soul, and with all your mind, and with all your strength.' the second is this, 'You shall love your neighbor as yourself.' There is no other commandment greater than these."
The apostle St. Paul reminds us of this: "He who loves his neighbor has fulfilled the law. The commandments, 'You shall not commit adultery, You shall not kill, You shall not steal, You shall not covet,' and any other commandment, are summed up in this sentence, 'You shall love your neighbor as yourself.' Love does no wrong to a neighbor; therefore love is the fulfilling of the law."
Source: Catechism of the Catholic Church (Vatican.va)