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."