Good question/topic.

I think, as others have mentioned, there's the "Us and Them" thing going on. Back when we were all members of little family tribes, fighting for food and resources with other little tribes, we needed to worry about "us" and protect ourselves from "them". We needed everyone in our "us" even if we didn't really like or relate to them, because they served a purpose in the "us"; better or specific skill set, (fighter, thinker, cook, caretaker...) or had a specific relationship with another important member of the "us". And that might be the answer you're looking for.

But if we dig a little deeper, we all want to be a part of a bigger "us". We want to be a part of something larger than ourselves. We possibly feel like we are "them" (or, in high school terms: unpopular) until we get in with the "us"(or, in high school terms: popular). This may be why people join fraternities, religions, social groups, online groups. (This website?) I think we all want to feel like we are part of a bigger "us", because we're then connected in some way that validates us (beliefs, interests, heritage, abilities/disabilities...), because we don't want to feel alone, and maybe because we hope they'll have our back.

Personally, while I fall into this patters as well, I do my best to be a part of, and to encourage the more global "us", but I don't discount the need and benefits of the smaller, more localized "us".

Just my thoughts.

((Edit/add: can there be an US without a THEM?? is this concept so ingrained in our society that we need a THEM so much that the US can't be The Human Race until Aliens show up?? Hmm - I wonder...))