It could be love thing, maybe just something that you do not want heard... Letters make secrets more smoothly said. For some, it's less embarrassing, less of a haggle to find words... Letters make emotions come out on paper, escaping the reality of face-to-face reactions.
Have you ever noticed that sometimes you can't tell someone your feelings in their face? Have you ever noticed the nervous, anxious feeling of telling someone a deep secret that you never even said aloud? Forgetting love, how about times when you just can't tell someone something due to their reactions? Or the words won't come out right? LETTERS... Well they saved me a lot of times.
"So many things to say,
I'll put 'em in a letter...
Thought it might be easier,
The words might come out better."
Anyone ever wrote out secrets on letters because of confrontational guilt? How about repressing feelings that you can't seem to say right? LETTERS... Well they can relieve a lot of us.