Why must people descriminate against unseen illness?

Guess what I'm 21 you can't see that I'm ill but it doesn't mean I am not. I have arthritis throughout my entire body and I have depression. No I do not show my pain, but it is there. I have a service dog that travels with me to help me with tasks you can do with no pain. People think I play the system to have my pet when they look. Don't judge a book by its cover.