ORIGINS of METTA practice
The Pali word metta means loving-kindness, friendliness, goodwill, benevolence, fellowship, amity, concord, inoffensiveness and non-violence. The Pali commentators define metta as the strong wish for the welfare and happiness of others (parahita-parasukha-kamana). Essentially metta is an altruistic attitude of love and friendliness as distinguished from mere amiability based on self-interest. Through metta one refuses to be offensive and renounces bitterness, resentment and animosity of every kind, developing instead a mind of friendliness and benevolence which seeks the well-being and happiness of others. True metta is devoid of self-interest. It evokes within a warm-hearted feeling of fellowship, sympathy and love, which grows boundless with practice and overcomes all social, religious, racial, political and economic barriers. Metta is indeed a universal, unselfish and all-embracing love.
Metta makes one a pure fountain of well-being and safety for others. Just as a mother gives her own life to protect her child, so metta only gives without conditions. To promote one's own interest is a primordial motivation of human nature. When this urge is transformed into the desire to promote the interest and happiness of others, not only is the basic urge of self-seeking overcome, but the mind becomes universal by identifying its own interest with the interest of all. Thus one also promotes one's own well-being in the best possible manner.
Metta is the protective and patient attitude of a mother who forbears all difficulties for the sake of her child and ever protects it despite its misbehavior. Metta is also the attitude of a dedicated friend. If these qualities of metta are sufficiently cultivated through metta-bhavana -- the meditation on universal love -- the result is the acquisition of a tremendous inner power which preserves, protects and heals both oneself and others.
Apart from its higher implications, today metta is a pragmatic necessity. In a world menaced by all kinds of destructiveness, metta in deed, word and thought is the only constructive means to bring concord, peace and mutual understanding. Indeed, metta is the supreme means, for it forms the fundamental tenet of all the higher religions as well as the basis for all benevolent activities intended to promote human well-being. Read the full metta sutta here.
source: http://www.accesstoinsight.org/lib/bps/wheels/wheel365.html#intro