Love letter

A love letter is a romantic way to express feelings of love in written form. Whether delivered by hand, mail, carrier pigeon, or romantically left in a secret location, the letter may be anything from a short and simple message of love to a lengthy explanation of feelings. Love letters may 'move through the widest range of emotions - devotion, disappointment, grief and indignation, self-confidence, ambition, impatience, self-reproach and resignation'.

https://en.wikipedia.org/wiki/Love_letter