Why do guys who get One Girl! Change who they are?

Long story short, I started at this sixth form late, made friends quick because I socialise well, there was one girl in the college that I had an eye for. However whilst befriending people further I found out that a friend of mine liked her as well. I presumed it was just a momentarily thing as it was for me, so we kind of left it there and just became good friends, then I was down one day and he was like whats up, you can tell your best friend, but it was nothing big. So few months pass and as I'm the observant type I could tell that he was falling in love with her not just a like thing, so I asked him, but he was too shy to make a move, so I was kidding with him and told him he has a 3 month window, to make his move or I would take her off his hands, it was a bit cocky I know but it was just to push him, because I also saw that she liked him too, but I knew there was no way she was going to make the first move, which is why I pushed him. He was pissed when I told him about the 3 month window but he didn't show how pissed and when I made one little joke, a racy joke but there was no offense behind it or nothing just a friendly one which he found funny, but then when we were in class he started arguing with me, so in my defence I argued back, I got angry because he wasn't making a point about why he was arguing, so when he started saying stupid things I replied with even more stupid things. After that we didn't speak for ages, then another friend of both of ours took the piss out of him, however he forgave him the next day. Then the he started taking the piss out of this guy who he forgave infront of the girl he liked, in attempt to show off, and the third friend let him have it because he knew he was trying to impress her, the second time he did it he managed to push him to the flower which naturally pissed him off and he retaliated by throwing him at the lockers and was about to punch him cause he pushed him to the floor hard and randomly until some guy stopped him, yet he forgave him again, which I now know for a fact why, because this other friend of ours isn't interested in the girl, but he thinks I am. So he's scared of losing her to me, but I told him, I said I'm sorry for what I said, I only did it for your benefit, and he gave me some more bullshit like it was some other reason, now I don't want to embarrass him infront of her, but why is he gonna get rid of a friend who stood up for him when other guys tried to bully him for some girl who he can never do anything with because she's pretty much wearing a chastidy belt that's not open to any guy until her dad says so.....