The rest of us could be covered with iron cast shields, but some sore spot always exists in us, which is fantastic. That sore spot is known as embryonic compassion, potential compassion. At least we have some kind of gap, some discrepancy in our state of being, which allows basic sanity to shine through.... Not only that, but there is also an inner wound, which is called tathagatagarbha, or buddha nature. Buddha nature is like a heart that is sliced and bruised by wisdom and compassion. When the external wound and the internal wound begin to meet and to communicate, then we begin to realize that our whole being is made out of one complete sore spot altogether. That vulnerability is compassion. We really have no way to defend ourelves anymore at all. A gigantic cosmic wound is all over the place -- an inward wound and an external wound at the same time. Both are sensitive to cold air, hot air, and little disturbances of atmosphere which begin to affect us both inwardly and outwardly. It is the living flame of love, if you would like to call it that.
-- Chogyam Trungpa Rinpoche, From: TRAINING THE MIND AND CULTIVATING LOVING-KINDNESS, Shambhala