Interdisciplinary Psychology:

You are right about the assumptions we make about our relationships. A lot of the time we don't even realized what it is we really appreciate about a person until you're kind of forced to sit down and articulate it in concrete terms. So often we show our appreciation of people with a "you're the best"  or something equally vague, which is still nice but doesn't really mean anything. It sounds like your friend really appreciated the letter.