Why do some dog have a fascination with peeing on plastics bags? Is there anyway to prevent this other than not accidentally leaving it on the floor. My dog is house trained, but if I was to leave an empty shopping bag on the floor, 90% of the time when I come back to pick it up an later, I would fi

This may be the most effective way to prevent head injury in yourself, and it is an unexpected benefit of dog ownership.  Plastic bags on the floor are dangerous slip-and-fall causes.  Let your dog make you keep your bags picked up. In our house, BTW, it's the cats that pee on the bags, not the dogs.

Peeing, in primal terms, is staking a claim, so your dog is simply claiming plastic bags. (BTW, dogs are usually scent-oriented. We may never know what smells coming from the plastics are triggering the peeing behavior, but it's not unreasonable to assume that there's some urine-related smell, or some urination-triggering related smell being effused from the plastic that's precipitating the peeing process. Plastics are an extremely complicated science. Ph.D.s are bestowed on the topic. We really know little about plastics, their byproducts, their off-gases, or their effused particle's effects on us or our pets.) There's an exercise you can repeatedly do to communicate to your dog – in dog language – that that bag, and all bags, are  yours and that he shouldn't think of claiming it, or peeing on it. If your leadership position with your dog is strong, next time he gets close to that plastic bag, a brief "hey," a hiss, a clap, or some other attention-getting behavior on your part will be all it will take to get his attention and let him know that that plastic bag is yours, not his. His response is critical, though. The way he responds will dictate what you need to do to guarantee success or failure. With a few successful repetitions, he will get it. If not, more (human) training may be necessary.

Dogs love crinkle sounds, not just bags or plastic.  He likes it so much he's trying to claim it as his own by peeing on it.  He likes it so much that you're lucky he doesn't eat them and risk getting clogged up.  You should definitely not leave plastic bags on the floor unsupervised.You can try giving him something different to play with instead and make sure he knows it's his.  There are lots of dog toys that make crinkle sounds, such as plush toys with an empty water bottle inside.  These should also not be left unsupervised, but you can give it to him whenever you're around and he'll love it.  There are more durable ones made out of Kevlar and similar materials.