Why is my puppy dog aggressive?

Every time my 4 month old german shepherd puppy see's a dog when we are going for a walk her hair on her back and shoulders stick up and she barks/growls very loudly. So I hold her back from the other dog to ovoid a potential fight.Since she is still a puppy the problem is still fixable but I dont know how to fix it. Or how to train her not to do it.

do you get nervous around other dogs?
she may feel your nervousness and wants to protect you
also she could just be really protective over you in general

also, she could have not been associated with dogs a lot, so she doesn't know what to think of them

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