Advice for my grandsons.
Why? As a show of friendship and trust. Also, as a show of affection to allow a boyfriend or girlfriend access to their most personal information.
Let me tell you that's a foolish thing to do, and it could lead you into trouble that could be embarrasing or worse. It could involve you in a serious situation.
What if you and your "friend" get into a disagreement or have a bad break up? Before you have time to think about changing your password, that "friend" could send emails in your name, post messages in your name, or send text messages in your name. If you think it can't happen, you are wrong. It can. It could. It might. It has.
Besides, what you write should be private. Keep your private life private. If you have a problem saying "no" to requests from people to share your passwords, just blame it on your parents. Tell your friends your parents would "skin you alive" if you shared your passwords. Be smart! Don't do it!