Metta — primary subject of the book Lovingkindness by Sharon Salzberg — is described beautifully in Chapter 2 ("Relearning Lovliness"):
No external condition can prevent love; no one and no thing can stop it. The awakening of love is not bound up in things being a certain way. Metta, like the true nature of the mind, is not dependent; it is not conditioned. When we practice meditation and perceive this quality of mind, we also contact the essence of metta. This produces a tremendous change in perspective. At first it is as if we were sitting on the shore and watching waves dance on the surface of the ocean. Later in meditation it is as if we are under water, in the calm, still depths, watching the waves above us moving and playing. Still later we perceive that, in fact, we are the water, not apart or separate, and that waving is happening. This is also how metta embraces all.