Letters can be written to friends for a variety of reasons, including to offer congratulations or thanks and to share personal news. These types of letters are called personal letters and serve as a form of communication.
In writing a letter to a friend, it is important to focus on sharing personal details about one's life and recent happenings that may be of interest. Some people will even include commentary about recent news events.
While it is not necessary to use a big event as a reason to write a friend, this can be the focus of the letter as well. As it is meant to serve as a written conversation, the letter should include feelings about particular topics in order to make it feel like the person is speaking with the writer.