Why is my boyfriend protective of this girl?

Anonymous
I've been dating my boyfriend for 8 months. He is very protective and very serious in the relationship. (We're 16)

A few months ago though I found out he's been flirting with this girl a year younger than us often behind my back. They would go on each others' profile on facebook and write messages. For example, my boyfriend once went on her profile, when through her picture and wrote "I want to be with (his name) forever he is my best friend :D" in the caption. Even before I found that out, we went on a one week vacation together with our families. He prevented me as much as he could from texting while he did. When I looked at his phone he had been texting her saying things like "don't worry I got your back :)" ect.

I got REALLY angry at this and I took a break from him. He kept begging to get back with me and how he only wants to be with me. He told me he only wanted to help her with her boyfriend problems. So, she can get one of her girl friends to help her out, why does it have to be my boyfriend?

Today I noticed that he has messaged this guy telling him to **** off and not to talk to that girl like that ever again or he is going to beat him up. I guess that guy talked inappropriately to her? I'm not sure what happened, that's all I know. He asked the guy to fight him and encouraged the guy too.

Why is he being so protective of her?
Why is my boyfriend protective of this girl?
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