For some reason, we can't just enjoy somebody else's success. Somehow, that's going to affect us. If they have more, then I have less - and I don't know why.

This quote refers to the feeling of jealousy or envy. It means that when someone else does well or succeeds, it can make us feel unhappy or bad about ourselves. We tend to think that if someone else is doing well or has more than us, then it means we have less or are not doing as well. This can make us feel jealous or envious of the other person.