Relationships

Why would My best guy friend asked me to keep our friendship a secret if we ever start dating other people. He wants us to be life long friends?

We have been friends for over 10 years. We have slept together but not on a regular basis. It didn't ruin our friendship. He says he doesn't want a relationship bc he is scared to lose me and says he wants to keep me as a life long friend. I guess I don't get it because friends break up too and become distance. I also believe that sometime partners (boyfriend/girlfriend) don't want their partners to have a best of the opposite sex it can get weird. Especially good friends that have an attraction to each other. For me, I would never secretly talk to someone behind my partners back. So keeping him a secret is not an option for me. Anyway I don't understand why he would want to keep our friendship a secret and also why he wouldn't just want to date me rather than lose me to someone else. Any thoughts from your past experiences?
Why would My best guy friend asked me to keep our friendship a secret if we ever start dating other people. He wants us to be life long friends?
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