How do guys feel when a girl tends to look at him and smile and suddenly one day she doesn't look at him and acts like he doesn't exist even when?

How do guys feel when a girl tends to look at him and smile and suddenly one day she doesn't look at him and acts like he doesn't exist even when he's around? and what would you do
How do guys feel when a girl tends to look at him and smile and suddenly one day she doesn't look at him and acts like he doesn't exist even when?
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