The kindness of the mother is chosen as the example of the intensity of kindness that all beings have shown us, because generally in samsara the mother's concern is something very strong and obvious. We can see the kindness of a mother not only in humans but in animals. A mother dog will starve herself to feed her pups and will die to protect them. In the same way, even if our mother were strange to us in some ways she still would have instinctively and unconsciously shown us great kindness. All beings have loved us in this very same way, sacrificing their food for us and even dying to protect us because their love for us was so strong. The people who are friends, enemies, and strangers to us in this life showed us the great kindness of a mother in countless previous lives.
Actually, there is no imperative that mother love be the model used here if doing so should cause a problem. If we have serious problems in our relationship with our mother, or if we were orphans and an aunt or uncle brought us up, we could just as easily take whomever we feel has been most kind to us and use him or her as an alternative model. We then contemplate how all sentient beings have been born into this same relationship with us in countless previous lives and have shown us these same kindnesses. We have to learn to see all sentient beings in that person's image.
As a result of this meditation one gains a feeling of spontaneous familiarity with all other sentient beings, a recognition that they are somehow very close to us and very precious.
--from The Path to Enlightenment
