How to make a dog, dog friendly?

Recently I have adopted almost 4 year old Maremma (family dog from puppy age & not flock guardian as some of them are, also she haven't been socialize much with other dogs by her previous owner). She is friendly with kids, adults etc.... But every time I taker her for a walk to the park I have to make sure there are no other dogs there. She is not what I would call a dog friendly. If the dog run to her I have to hold her while they do sniffing part, but then she might attack it or get a bit aggressive. I do have second dog and she is fine with her (could be a bit ruff while playing though). What can I do to stop this (dog unfriendly) behavior, and improve it? Is there a way?