Really though where did this brotherly love stuff come from? It's never really been Philly.
Speculating but I think it stems from a "If you're not with us, you're against mentality" here. If you're with us, it feels like a big family and it's all love. If you're against us we hate you. This is just coming from an Eagles fans perspective, not a person who lives in Philly. I'm from South Jersey and whenever I wear my Eagles jersey and a lot of times when another Eagles fan sees me we cut it up like we're best friends

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It has absolutely nothing to do with eagles fans chopping it up and having an us against the world mentality
The name Philadelphia itself roughly translates to brotherly love from Greek, which was given by a Quaker, William Penn.
Edit: and sorry but that's such a dumb mentality to have, "if you're not with us, we hate you"
It's funny how many people in this thread have come to Philly's defense, yet embodied literally every negative stereotype