The word "dear" itself is used for many kinds of relationships, and it often does not translate well to other languages. It can be used casually or intimately, the difference lying in context. Letters are often addressed to "Dear So-and-so", where no relationship exists at all. So do not take the word too seriously.

Good question. It is usually used in formal letters, exampple: "Dear Sir", "Dear Mrs ______" But can also be used for affection

Najia my dear, as Randy informs us, there are many uses. And before addressing your question further, may I inform you that I think Randy is such a dear, because he writes so many excellent answers in the italki forums, which as you know, are dear to us all. One of my dear students, is a person with some evidence of Suicidal Tendencies, so I often refer to the student as my dear friend, and remind them that I care for them. Others of my students happen to be of certain Astrological Signs which are harmonious with my own, and as a consequence, they are especially dear to me because I feel so much love for them. My dear wife sat beside me at a baseball game tonight, while my dear grandchildren played nearby. Their dog "Jack" is very dear to us all. I have a brother who is not so very dear to me at all. He and I have corresponded only a little in many years, and I have to take great care of his cunning manipulations. You know? Like he might refer to me as a "dear" brother or say that he "loves" me, but he doesn't really. I have not mentioned the game of Chess, which is very dear to me. Philosophy is dear to me also. Almost all of my language students are dear to me. I had a dream recently about a beautiful girl, and I held her tenderly in my arms, kissed her head and whispered the word "dear" into her ear. . --Hope this helps. .

Often, old people are called 'dear 'by shop assistants.