From Lama Zopa Rinpoche (Transforming Problems Into Happiness):
"Simply by rejoicing, without the need for extensive preparation or any special effort, you can accumulate incredible merit, incredible good karma. In one second you can accumulate merit infinite as space. If you really practice it honestly, rejoicing in others is the easiest way to accumulate extensive merit and attain ultimate happiness... The practice of rejoicing can prevent the arising of pride, jealousy, or anger. Whenever you hear that someone else has been successful, rejoice. Always practice rejoicing for others - whether your friend or your "enemy" - even about samsaric success. For example, when you hear that someone has been successful in business, you should rejoice. When you hear that someone has found a partner, again rejoice. You should think "How wonderful it is that they have found the happiness they were seeking!" Practice feeling as happy for others as you do for yourself. In other words, cherish others as you cherish yourself. Feel happy when others find happiness, as if you yourself had found happiness. If you cherish only yourself, you cannot experience happiness, but if you cherish others as you cherish yourself, happiness arises naturally... without any need to think of reasons. When something good happens to anyone, you naturally feel happy. Every positive trait is the result of past good actions, and all can be a basis for you to rejoice. When you see someone possessed of physical beauty, think: "How wonderful it is that this person has a beautiful body now as a result of practicing patience and morality in past lives."... If someone is very intelligent or has more understanding of the Dharma than you do, you can rejoice in it as well."
I think this is a very good form of practice. Something to keep in mind
But sometime, it is not easy to rejoice with sincere intention.
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I went to Thich Naht Hanh talk the other day at Kong Meng San. I think his teachings on interbeing is very good.... the part where he said something like a piece of paper is paper because of the conditions of the tree, sun, rain etc., and a person is person because of father, mother, food, etc.
This mindset connects everything up in a big web, and that "we" are "we" only because of the supporting conditions of everything else - other people, other creatures, the trees, the mountains, the sun, the earth, etc. And in turn ourselves are a condition for everything else as well. If we think in such a manner, then if people are well and happy we will be able to rejoice more easily.
yes, since self is basically an illusion and there is really no separation between me and others, there is really no need to be jealous of others' achievements or well-being... rejoicing in their wellbeing is a very powerful antidote against jealousy and competitive grasping mind