Why do I guys treat me differently?

I'm in High School doing my final year. Everyone gets along great, there's a lot of banter and joking between everyone, we're like one big family. The guys usually have their arms around girls in a friendly way though and it seems like everyone is at ease. But it's different with me guys don't treat me like other girls ( Not that I want them to hang their arms on my shoulders, just the sense of belonging). I don't know if I make sense, I hope I do.

Anyway I noticed that guys seem more reserved around me or show a watered down version of themselves. I want people to feel free around me and this is really bothering me, I feel left out. Even the school douches act this way. And it's mainly guys as with the girls I get along well with almost every girl and they don't act weird.

Please give me an idea on what may be the cause.

Why do I guys treat me differently?
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