In my post below, Tolkien and Marriage, the section quoted brought up other odd thoughts. Who talks like this to their children? I would come closer than most to that style, yet even I found it puzzling. (Not many other children are admonished as they leave the house "Go forth. Do well. Be of good cheer.")
My first thought was that this was the product of a different era, and that was sufficient explanation for all. Yet that put me in mind of writing letters to one's sons at all. Who does that anymore? I write emails, blog posts. Everyone picks up the phone much more readily these days.
There was a short period of time when I wrote many letters, actually corresponded with people, as a young man. But the writing of letters to intimates is now long gone. I imagine there are some die-hards, determined to stick with the form of communication they do best and mindful of preserving the art as long as may be, but there can't be many now. We briefly returned to writing letters during Chris's three months at Parris Island last year, and I did get more formal. That is perhaps notable because Chris is the least interested in reading of all the sons; so the form of communication itself directs us to write things we would otherwise not. We are very proud of you. How are things spiritually with you? A letter, being infrequent and limited in space, invites one to cut more quickly to central issues and bring them out into the air.
Were I writing to Jonathan or Ben, in a situation where it was all the communication we would have that month, I would move easily to such topics as marriage and choice of mate, and it would seem less strange to them to receive it.
Bearing in mind that when Tolkien wrote to his sons, he did not expect that others, especially the general public, would also read them, his tone is even less surprising. Confidences that seem impolite to mention, such as his annoyance with several of the Inklings, or his prejudices against groups of people, were mentioned precisely because they were in confidence. If someone were to publish my collected emails to my sons and others close to me, there are certainly more than a few I would want back immediately for editing, if not permanent electronic destruction.
Come to think of it, I'm sure there were letters like that which never made it to publication in The Letters of J.R.R. Tolkien. The editors have considerable discretion in such matters, and we do not see the entirety of a person's correspondence. Yet even at that, the letters which are published were never meant for another audience at all. If one traveled in time and spoke with Tolkien (which I do, occasionally - usually around 1941), mightn't he feel seriously intruded upon if I were to refer to knowledge which could only have been gained from the letter to his son (or wife!) he had on his desk that very moment? Wouldn't all letters written after have an eye over the shoulder to posterity about them?
I have learned more than I would like about Tolkien's dislike of Americans or irritations with Lewis. But I am applying a standard of public statement about those matters that is not fit for letters to his children. I make less-guarded statements to my children all the time, things I would forbear mentioning on the internets.
So, different era; different medium, not by election but by necessity; therefore, very different things communicated.