Why is my best male friend pulling away from me?

RedSmartie
My best guy friend and I are usually really great friends. Until recently we would hang out at each others' houses a lot and spend our breaks during school together.

We've both been taking two six-week summer courses Monday through Thursday at school (they end tomorrow), we're in different classes but we carpooled and would spend our breaks together.

But for the past three weeks he has been dropping off contact. It started off with just the breaks between classes, I went to the usual meeting spot and he started showing up less and less. Then we stopped hanging out after school, I would just drop him off at his house (or he would drop me off at mine) and then he'd say he had to go do homework or whatever, which for all I know he might because he has TOUGH classes, but we used to spend a few hours playing video games after school to relax before studying, but not anymore. And now he doesn't even want to carpool. I was waiting outside my house yesterday for a while before giving up and driving myself to school. I texted him asking if he was okay, and he replied just saying "yes" with no explanation.

I've been seeing him around campus with this girl a lot lately (it's a small campus, I'm not a stalker) and I was thinking it might have something to do with her, but if it is related to her then I'm really confused. The last time he showed an interest in a girl, he didn't stop being friends with me. We just carried on the same way as normal except we'd talk to her sometimes too.

I can't figure out why the new one is so different that he feels like he has to cut off contact with me entirely.
Why is my best male friend pulling away from me?
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