The shadow is basically a hidden part of someone that is projected onto other people. Most of the time when someone dislikes something about someone else its a suppressed part of themselves.
For example if their parents yelled at them for doing something as a kid and all their life they grew up seeing kids do something they truly wanted to do, but judged them as doing something wrong. Then as an adult that part of them gets more and more suppressed as they justify what their parents taught them even though its a secret part of them.
Everyone does this and its not just the things their parents taught them its in our culture. What I'm trying to encourage is that instead of dwelling on disliking someone for doing something, dwell on why you dislike what someone is doing and figure out why it is that your not doing it. Maybe it isn't even a logical reason.