The Karaniya Metta Sutta goes on to say that when you’re developing this attitude, you want to protect it in the same way that a mother would protect her only child.
As a mother would risk her life
to protect her child, her only child,
even so should one cultivate a limitless heart
with regard to all beings.
Some people misread this passage—in fact, many translators have mistranslated it—thinking that the Buddha is telling us to cherish all living beings the same way a mother would cherish her only child. But that’s not what he’s actually saying. To begin with, he doesn’t mention the word “cherish” at all. And instead of drawing a parallel between protecting your only child and protecting other beings, he draws the parallel between protecting the child and protecting your goodwill. This fits in with his other teachings in the Canon.
http://www.accesstoinsight.org/lib/auth ... odwill.pdf
Thanissaro Bhikkhu (continued) wrote:Nowhere does he tell people to throw down their lives to prevent every cruelty and injustice in the world, but he does praise his followers for being willing to throw down their lives for their precepts:
Just as the ocean is stable and does not overstep its tideline, in the same way my disciples do not — even for the sake of their lives — overstep the training rules I have formulated for them.
— Ud 5.5
The verses here carry a similar sentiment: You should be devoted to cultivating and protecting your goodwill to make sure that your virtuous intentions don't waver. This is because you don't want to harm anyone. Harm can happen most easily when there's a lapse in your goodwill, so you do whatever you can to protect this attitude at all times. This is why, as the Buddha says toward the end of the sutta, you should stay determined to practice this form of mindfulness: the mindfulness of keeping in mind your wish that all beings be happy, to make sure that it always informs the motivation for everything you do.
From: Metta Means Goodwill by Thanissaro Bhikkhu