Being fair
Your friend tells everyone something that you told them in secret. Is that fair?
No. If they knew it was a secret
You and your house mate bought a pack of 6 chocolates. She eats 4 before you get to them. Is that fair?
No. If they belong to both of you, it's fair to share.
Your friend orders a pizza. You say you don't want any. When it arrives he refuses to share. Is that fair?
Yes. You said you didn't want any