"You shall love your fellow as yourself" (Leviticus 19:18). Hillel the Elder called this the entire Torah, with everything else being commentary. Chapter thirty-two of the Tanya explains how to actually do it.
The key, says Rabbi Schneur Zalman, is which part of the other person you are looking at. If you focus on bodies—his body versus your body, his interests versus yours, his status versus yours—love is impossible. Bodies are separate. They compete for the same resources. Every gain for him is a potential loss for you.
But if you focus on souls, the picture reverses. All Jewish souls share a single root in the one God. They are, in the Tanya's language, "all of a kind, and all having one Father." Siblings from the same parent. The more weight you give to the soul and the less weight you give to the body, the more natural it becomes to love the other person—because at the soul level, there is no "other." You are one.
This is why people who prioritize their physical existence cannot sustain genuine love. Their love is "dependent on a thing" (Avot 5:16)—conditional, transactional, subject to change when circumstances shift. But love rooted in the shared divine source of all souls is unconditional, because the source never changes.
The Tanya addresses an obvious objection: what about people who sin? Aren't we supposed to hate the wicked? Yes—but only under specific conditions. The obligation to hate applies only to someone who is your "companion in Torah and commandments," whom you have personally rebuked, and who has refused to change. For everyone else, Hillel's instruction applies: "Be of the students of Aaron, loving peace and pursuing peace, loving people and drawing them near to Torah." Even total strangers. Even people who seem far from observance. The Tanya says you should draw them close with bonds of love—and perhaps the light of your love will inspire their return.
At the cosmic level, this love is not just an ethical imperative. It is a structural necessity. God's light can only rest in a unified vessel. Disunity among souls creates cracks that the Shechinah cannot inhabit. The prayer says: "Bless us, our Father, all of us as one." Only as one.