Asking why someone writes, or wrote, is a tricky question. I am a writer and I don’t always have a good answer to this question. When we start asking the question about people who have been dead for any length of time, let alone hundreds and hundreds of years a satisfactory answer may simply not be possible.
St. John was writing in the first century. He is the only eye-witness to Jesus who wrote in the New Testament, at least so scholars believe. These two facts taken together make John’s writing very interesting to us while being behind the curtain of another time, place, and culture.
I am not about to suggest John had only one reason to write but if he did, I am saying it was to let us know love is the most important thing because God is love. Let me know what you think.