Lama Zopa Rinpoche explains:
Cherishing others means that you don’t harm others, and not harming others is not harming yourself. Even in terms of protection, this is the best way to protect your life. Similarly, when you cause others to be happy, you bring happiness to yourself. The karma created by making others happy causes you to experience happiness too; that’s the kind of karma that results in happiness. Even if you don’t want happiness, once you have created its cause, that’s what results.
If you plant a seed in the ground and all the right conditions are present, such as perfect soil, water, and heat—every thing is together and there are no obstacles—then no matter how much you pray for the plant not to grow, it will grow. It will definitely grow because the seed planted in the ground has met all the conditions necessary for growth; the cause and conditions have met. Since it is a dependent arising, it is inevitable that that flower or fruit will grow, no matter how much you pray for it not to.
Similarly, if you lead your everyday life with compassion, bringing as much happiness to others as you possibly can, the natural result will be for you yourself to experience happiness, both now and in the future—there’s the immediate effect of peace of mind in this life and the long-term effect of happiness in all your future lives. All this is the definite result of bringing happiness and benefit to others.
Lama Zopa Rinpoche
from The Joy of Compassion