Why does my guy friend feel the need to remind me the we are just friends?

I’m been friends with this guy for three years (he is my best friends’ boyfriend). I am not attracted to him (like at all) and he’s not my type. I ask him for guy advice and talk about guys I like (something that I don’t do with I guy I was into). I’ve noticed that throughout our friendship, he has felt the need to constantly remind me that we are “just friends” and that he doesn’t see me in that way. I’m not doing anything that crosses boundaries or anything.

Why does he feel the need to do this?
Why does my guy friend feel the need to remind me the we are just friends?
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