We feel true Love for others when we recognize at an experiential level (rather than an intellectual level) that there really are no boundaries between us -- that each of us is simply a different "view" into Consciousness. It goes beyond the concept that we are all "One" -- rather, there are no individuals to unify -- there is only Consciousness. When we directly experience this, all the threats and conflicts we fear from others disappear. The absence of these sensations leaves only the sense of Oneness, which is the sense of Love.
Spiritual experiences (ecstatic, satori, etc.) all come from a direct experience of the unity of everything.