Why would a guy suddenly start ignoring a girl?

so I've been talking to this guy as friends, he is just really friendly...i have not flirted with him I'm just friendly also, we joke and have fun, I admit I do like him and have a crush on him but I have not made a pass at him or anything, I Haven't flirted at all I talk to him like I talk to other girls, and lately he has been ignoring me and I smiled at him and he just didn't even pay attention...? I don't understand what happened?
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i mean I didn't wanna be too forward ya know? I always approach him and smile and say hi, I make jokes...but I thought that he friend zoned me...so I didn't flirt after that, we were still cool but now he's completely ignoring me. and I'm certain he had friend zoned me cause he called me sisterly
Why would a guy suddenly start ignoring a girl?
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