Well, I am not an expert but, my vet said that all dogs in New England will likely test positive for Lyme. This means they have been exposed but, it does not mean they necessarily have the disease.
My dog had Lyme, the disease, and got better with 6 weeks of Doxy. He will test positive for the rest of his life. I, personally, would not be trusting this vet and treating my dog every year without evidence of active disease.
Antibiotics have their own risks, and unnecessary use should be avoided.