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Also known as the Brahma Viharas, I’ve been looking into them recently by making my way through past topics on NewBuddhist where they were talked about. Apparently @Glow made it his main practice for about 5 years. I came across this:
The brahmaviharas are best seen against their opposites:
metta (goodwill) is the opposite of hatred or ill-will,
karuna (mercy/compassion) is the opposite of cruelty,
mudita (appreciative joy) is the opposite of resentment or jealousy,
upekkha (balance or equanimity) is the opposite of craving.
These qualities are about committing to the end of suffering. If you’ve worked with the defilements (kleshas) and have come to a point where further insight is yielding you equanimity but not happiness or bliss, then my experience is working with the brahmaviharas will allow you to advance further in the path. If you feel peace and happiness when you do the meditations or contemplations then it is probably doing you some good.
It seems to be important to approach the brahmaviharas in a balanced way (see this older discussion), to me it makes sense that too much loving-kindness and compassion practice might lead to increased sensitivity to suffering, whereas appreciative joy moves the center of the experience away from yourself and equanimity practice encourages a measure of stability. Personally I started with appreciative joy as something I felt I needed to experience.