Goodwill for yourself means not harming others, and then you start thinking about their happiness, too

"Another guardian meditation is goodwill [mettā]. This is for times when you’re feeling angry at other people. You have to remind yourself: If you want safety in this world, you have to give safety to others. This is one of the reasons why we observe the precepts. No harming. Period. And as the Buddha said, when you’re resolved to be harmless to all in line with the precepts, with no exceptions, when your virtue is universal, then you have a share in that universal safety. If the safety you give to others is partial, then your safety is partial, too.

So you want to learn to see your anger and your aversion as dangers within you. Then you develop goodwill for yourself. You don’t want to inflict those dangers on yourself and you don’t want to inflict them on others. So you develop lots of universal goodwill, spread thoughts of goodwill around to everybody without exception.

The Buddha said you should care for this universal goodwill in the same way that a mother would care for her only child. In other words, you maintain it despite anything that anybody might do. So goodwill for yourself means not harming others. And then you start thinking about them. They want happiness too, just like you. It’s just that we live in this world where people have lots of different levels of understanding and levels of behavior, and you have to be forgiving. So when anger comes up, you’ve got a tool to deal with it.

Sometimes people say, “Well, will everybody truly be happy? When you say ‘May all beings be happy, may all beings be happy,’ will they be happy?” Even the Buddha didn’t answer that question. Someone asked him one time, “Would the whole world, or half the world, or a third of the world reach awakening?” He didn’t answer. As Ven. Ananda later explained to the person who asked the question, the Buddha’s main concern was that if people are going to find happiness — and it’s their choice — if they want to put an end to suffering, this is how it has to be done. But because people have free will, there’s no telling what the choices made in the future will be.

So the whole purpose of goodwill is not so that we think that someday all the world will all be happy. It’s because we’re taking responsibility for our actions, we’re not going to blame our actions on anybody else’s unskillful behavior, so we want to make sure that our motivation is right: that it’s harmless, both for ourselves and for other people. That’s the second guardian meditation."

~ Thanissaro Bhikkhu "Guardian Meditations"