I think it might have a lot to do with disposition, as others have mentioned.
My Dolly has been going to school for 4 years. She’s always been ambivalent to other dogs - she’s way more interested in people. There’s another, older, male Golden Retriever named Rex who’s been in school with us for about 2 years. All of the other (usually smaller) dogs jump on Dolly when they’re excited, and they bark. Rex is very laid back and never says a word to her. And he’s also not too interested in other dogs. In fact, he was rescued and used to be a mean dog. After some training he got better, and Dolly was the first dog he was around that he wasn’t agressive with - because she’s his size, and doesn’t jump and bark or get up into his face.
If you visited our class you might think that Dolly and Rex were buddies because they’re both goldens. But it’s really because they have very similar temperments and attitudes towards other dogs.
At my aunt’s house there’s a big mutt (possibly wolf hound and lab) and a crazy boxer mutt. Occasionally my cousin’s spazzy, barky golden retriever is there too. He’s the same size and color as Dolly and looks more like her than Rex (the one at school) and she can’t stand him. She can’t stand the crazy boxer either. Just the big, laid back dog.
I would LOVE to see her interact with a bunch of goldens though. It’d be really interesting to see how she interacts with a bunch of dogs that are about the same size and temperment.