Metta bhavana, Pali for 'the cultivation of loving-kindness,' is one of four brahmaviharas, or 'divine abodes,' in the Buddhist tradition. The four are metta (loving-kindness), karuna (compassion), mudita (joy in others' joy), and upekkha (equanimity). Together they are called the immeasurable qualities: states of heart that, once fully grown, have no edge and leave no one out. Metta is the root. It is the deliberate growing of unconditional goodwill toward all living beings, starting with yourself. You do it by generating and extending a specific warm, open-hearted wish: may you be happy, may you be healthy, may you be safe, may you live with ease. These are not affirmations or positive thinking. They are genuine intentions, aimed first at yourself, then widening in circles to a benefactor, a dear friend, a neutral person, a difficult person, and finally all beings without exception.
Most of the science on metta comes from Barbara Fredrickson's lab at the University of North Carolina, built on her broaden-and-build theory of positive emotions. Her landmark 2008 study, 'Open Hearts Build Lives,' published in the Journal of Personality and Social Psychology, followed 202 employees at a software company over 7 weeks. Half were randomly assigned to a loving-kindness meditation program. Those meditators felt more and more positive emotions over the study, not just while meditating but throughout daily life. Those positive emotions built up real personal resources: sharper mindful attention, a stronger sense of purpose, more social support, and fewer symptoms of illness. The gains held after the study ended. A 2013 study by Adrienne Taren and colleagues found that 3 weeks of loving-kindness meditation reduced bias against stigmatized groups, a result that matters for how people get along, far beyond any one person's wellbeing.
Every metta practitioner runs into the same wall: the difficult person. After the warmth of wishing yourself well, after the easy glow of sending goodwill to a beloved teacher or close friend, after the mild neutrality of wishing well to the clerk at the post office, the instruction comes to send loving-kindness to someone who hurt you, someone you resent, someone you find hard to stand. This is where the practice stops being pleasant and starts to change you. You are not asked to pretend the resentment is not there. You are asked to hold the difficult person in mind and genuinely wish them free from suffering, because a person who is causing harm is, by definition, suffering. Thich Nhat Hanh teaches that you cannot fully understand anyone until you know their suffering. The difficult person is difficult precisely because they are in pain and expressing it in destructive ways. Real metta toward a difficult person asks you, and helps you, to see that person differently.
At the deepest levels of metta practice, the boundaries of the self that holds and aims the goodwill start to dissolve. The line between the one sending loving-kindness and the one receiving it grows thin. What the tradition calls 'the divine abode' stops being a feeling you generate toward others. It becomes a quality of awareness itself: the unconditional goodwill that is the mind's natural ground before fear, desire, and aversion layer on top. Ajahn Brahm, the British-born Theravada monk who leads the Buddhist Society of Western Australia, calls this 'the opening of the heart-door': a shift from doing metta as a practice to resting in the awareness that genuine love is the mind's natural state whenever it is not squeezed shut by fear, desire, and aversion.