Seriously, I have to write this all out again?
First, read the ep 4 library scene again
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Try to put yourself in the MC's shoes. Do you think that he might have thought that there was a possibility of a relationship here? And how would you feel when this possibility of a relationship turn's out to be that you were an emotional crutch for this girl? And now that she has what she wants, you just don't matter anymore. Especially given that she never even told you that there was already someone in her life?
Josy did tell MC that she had someone else. He then did the right thing and told her this is farewell. But Josy insisted that it was not farewell, only good bye while crying and hugging him. Don't you think she was sending him mixed signal's here? Don't you think she was holding out the possibility that there could still be something between them?
Then later when she calls him at college it sounds very much like she's about to end her relationship and run away. What do you say then to a girl you've had a crush on all summer? 'OK Bye, have a nice life' Have you ever heard of the concept of drawing somebody forward by running away? Then, after their date together he's just about to leave when she interrupt's him and invite's him into her bed. What do you think you would be feeling at that moment?
The situation is pretty clear, and both girl's admit it, that they used the MC as an emotional crutch at a time when they were both sad and lonely. How would you feel in that situation?