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I think it's something like a Bound Gods tradition that was founded by Van. He never liked to be called Master. It's a question of taste. I really like the "sir" while in my own language "Herr" or "mein Herr" sounds quite stupid to me...
"Sir" is my favorite. I think this word is contain a lot of respect for another person.
Like saying Ma'am to a mistress. For you would never in proper English call her mistress to her face. Also I am sure you wouldnt call a Master "Master" but actually mister.