What's wrong with writing a book on yourself, a friend, or family member as long as you don't use real names?

If it's fiction , fine. But if it's not, and it is clear to others about who the characters really are, it's wrong.

well a friend of mine wrote fictional diaries in a female's point of view...considering he is male

Because they might get upset that their dirty laundry is broadcasted to the world.

Because they...ughhh. I can't even say why. chitshtichit.