Why would a guy randomly start ignoring you/being rude?!

I apologize in advance for the novel I'm about to write (: I work with this boy (he's only 17, I'm 19) and we have that playful, flirty we-tease-eachother kind of thing going on (stupid, I know), but lately he has completely stopped all of that and switched over to this arrogant-like jerk. I'm not... Show More