Guy's Behavior

Why is my friend who has called me his sister in the past being like this, I realize it's long, but please answer?

Hi, so I have been friends with this guy since 9th grade, we are now in 12th grade getting ready to graduate, but we are both staying home for college and live close to one another. We have always been good friends, but this year have gotten extremely close thanks to a class that we have together (he has even called me his "little sister" once before) so close that I have begun to have feelings for him. He has always told me that he doesn't want a high school relationship and wants to focus on his grades, which I understand, therefore he hasn't had a girlfriend. He's a genuinely nice guy all around and is always really flirty with me a lot more than his other "girl" friends, he plays with my hair a lot, I will catch him glancing at me in class, he touches me, etc. and so on. He came over to my house one night with his brother and was even leaning up against me while I played with his hair, so with this being said because I thought he felt the same way, and knew how I felt, I told him how I feel. He didn't really say anything back he just apologized to me, and I could tell he really didn't know I felt that way. Usually I would expect things to be awkward after this but after we talked about it, multiple times, our relationship really hasn't changed. So after that I started talking to this new guy to get over him, and my friend flipped and started drilling me with questions everyday about him and said he "didn't approve." The guy and I didn't really go well and when I told my friend he said "Sorry, so whens the wedding😂" Then the day after that I found out he asked a girl, who is one of his friends, and I know for a fact he is not attracted to her at all, asked her to prom)-:. We're on break right now so I haven't talked to him, every time we hang out its because I will put the effort in never him, I just don't understand. I realize this is long, and I appreciate it, thanks.
Why is my friend who has called me his sister in the past being like this, I realize it's long, but please answer?
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