People fall into the trap of thinking a friendly reminder email is professional and polite, but in reality it is a passive aggressive way of asking someone for something that is late. Usually those who receive these type of emails begin to resent the person who is sending them. Friendly reminder emails undermine the credibility of the person who is sending them because they soften the blow of what the person is really trying to say.
Read more: Everyone Secretly Hates Your “Friendly Reminder” Email