There are two girls here
1)Black haired girl - Hyoka
2)Blond Haired girl - Ayama
The black haired girl is a plain girl that started prettying herself after high school debut. As the Mc confessed to another girl and got rejected, he somehow started getting distanced by this said childhood "friend". Why? According to her, he left her for a popular girl. One of the most in el takes ever. The Mc didn't even see her romantically or really even attracted after she became popular. Why? Because to him nothing changed, she is the same narcissistic cold asshole to him as she was back in middle school.
As to why the Mc confessed, pretty fair point from Mc, she was touching him too freely, spend most of her class time together with him, and she saw him as a friend and rejected him. He then said if a girl showing this much closeness doesn't warrant interest then he is not interested in live. Again a very mature follow through by Mc.
Then comes the cliche blond haired girl was mistakenly assumed as a boy by Mc in early life. He met her outside of school before she transferred and she approached him, talks with him after class, eats lunch with him and doesn't berate him with insults every few seconds.
If I explained this to a blind man, he would choose Ayama. The Mc not understanding Ayama's intention is fair ground for his past experience and immature age. But Somehow if he ends up with hyoka(which might as well be with the cover and title), this author or Japanese people maybe has one of the most dogsht taste in women.