Here's a quote from the book (starting page 246) that I like:

Bobby continued: "I think that little children and little animals sometimes are smarter than adults. Know why? They can see good and bad hearts right away. Good hearts give love and food and hugs. Bad hearts give hits and hurts." Bobby said he thought that most people lost this ability as they became older. But he saw a ray of hope: A few adults are stronger inside. They don't give up their good hearts.

"Maybe," Bobby reflected, "it's because I'm still a kid. Maybe it's even because I've been hurt a lot. Anyway, I know right away, almost always, about who has a good heart. It's this light thing. A few people have strong inner lights. But most people have 'lights-out.' 'Lights-on' not only means a good heart; it also means really caring about other people's hearts.

"Now here's the main thing. Good is when people, no matter what their age, struggle to keep their lights alive. Good is when they help others with dim lights to make those lights brighter. Most of all, good is when they try to link up with other people who have good hearts.

"Okay, last idea. I think of the world as a huge mountain. But it's very dark and cloudy and smoky. You've got to climb the mountain. But it's all covered up and it's so hard to see. Even if you have a good light, it's not enough. You've got to find other people with lights. And when you get enough lights, then you can climb and not fall off. It's scary. You're not sure what's at the top. But with all the lights, you might get there. And even if you don't get there, you might help people on the way."