The exact reason dogs drink piss is unknown. This behavior has been seen in dogs that have a tract infection. The sickness causes them to become terribly thirsty, so that they can drink no matter liquid they will realize. The behavior might even be attributable to environmental reasons. If your dog's behavior is caused by a tract infection, then treating the infection ought to curb his piss drinking habit. However, if you're unsure why your dog is drinking piss, then he would possibly like re-training.