"Recently I caught my dog peeing in the house. He's never done this before, what's going on?"
Dogs do this for any number of reasons. If he has been properly house-trained he should not be doing this. He might have UTI (Urinary Tract Infection), in which case the vet should be your first stop, or he might be scent marking.
"What is scent marking?"
Scent marking is a completely natural thing for your dog to do. By peeing somewhere he leaves his scent telling other dogs that this is his territory. Remember that a dog's sense of smell is much better than our own, so for him a scent marked spot is a very interesting
place, while to us it just smells like dogpee!
"Do all dogs do this?"
Yes, they do. Some dogs are more likely to do so than others, males more than females, small dogs more than larger dogs, and so on. Two or more dogs in the same house may regard each other as competition and are then more prone to marking. Small dogs are also more likely to pee inside the house in general.
"Okay, I understand what marking is, but why is he doing it in the house?"
Your dog might be ...