Why is my guy friend so overprotective?

Okay so I like this guy and we’re pretty good friends, we’ve known each other since we were 9. I told him I liked him and he kinda changed the subject. Why is it that when I talked about guys I considered dating in the past he was so overprotective and found reasons for me not to date them? I’ve never dated anyone because I’ve had a crush on this guy since I was 10 (I’m 15 now) and I’ve tried to push it aside and start dating other people and since he’s my friend I ask him for advice and he always convinces me not to date them. What should I do because this is getting quite annoying.
Why is my guy friend so overprotective?
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