Why does my best friend think about killing himself?

I mean, I’ve thought about what life would be like if I didn’t exist, but it’s more in the curious “what if” state of mind. My best friend has thought about it more than I have, and I’ve been really worried about him lately. He’s finally getting help with it, but I just don’t know why he thinks like this. It’s scary, and I just wish I could understand why he would think so little of himself.