It is rude to disrupt someone's belongings without permission.
Sally asks John to watch her bag while she runs to the bathroom.
John wants to use some lip balm on his chapped lips.
John waits for Sally to get back from the bathroom and asks if she has lip balm.
John goes through Sally's bag to look for lip balm.
John and Sally maintain a good, mutually-respectful relationship.
John damages his relationship with Sally because she feels violated.