It is a good idea to write letters expressing feelings whenever the person is unclear as to how she feels about another person or situation, and this uncertainty is preventing her from functioning normally. A letter can be a way to process complex feelings and emotions and organize thoughts.
Sometimes a person needs to sit down and write a letter expressing his or her feelings about a relative, friend or romantic partner in order to work through a difficult situation. For example, if she is in a commuted relationship with someone and that person cheats on her, she is likely to experience a series of complex and confusing emotions. The hurt over the betrayal of trust stands in contrast to the feelings of love, and may be very distracting in her normal, day-to-day life. Writing a letter expressing the way she feels about her partner and the situation can help her to sort out the emotions and understand why and how she feels the way she does.
A letter expressing feelings is also useful when a person has unclear feelings of affection towards someone else. In this instance, a letter can help the writer work through the reasons for the affection, and gives her a chance to thoughtfully express why she cares about that person.