Men, why do you do the head nod to the girl?

personally I find it rude like you can't be bothered to talk to us. This one guy I kinda like, like I waved and smiled at him and before he would say like hi how are you, and stuff like that or ask how long I'm working. But today he just did the up nod smile thing and kept on walking and I didn't even see him for the rest of the day. I always take it as a "hi I see you but I can't be bothered to talk to you, or even say hi". its terrible.
Men, why do you do the head nod to the girl?
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