I used to wonder how this worked too, because it is an apparently selfish activity. The way I understand it now is: people are good when their minds asses that

I used to wonder how this worked too, because it is an apparently selfish activity. The way I understand it now is: people are good when their minds asses that they have what they need. Importantly, it's necessary to take a broad view of 'what one needs' here: resources in this sense range from tangible things like food and rest, to self determined personal inadequacies (that drive people to attempt making others feel inferior etc.). When peoples' minds determine resources are lacking, people are in a very different mental mode; their strategies are biased to take from others (whether the resource is tangible or not). So, the idea is that feeling you have everything you need is a prerequisite for true goodwill toward others; and, 'feeling you have everything you need' or rather believing, 'nothing needs changing,' is a common characterization of the sought after mental state in meditation. I've observed the difference in character of thoughts that emerge in myself personally when I'm relaxed and content versus the opposite, and I'd bet there's something to this Buddhist idea.