Remember when you house-trained that puppy? Remember how he went to a part of your home far from his bed to make an indoor messes? As a baby, he was only mature enough to regard his sleeping area as his den. But as he developed into an adult, he learned that his den was really your entire home. He has not needed more house-training, because his instinct not to soil the den matured right along with the rest of the dog.
So now this poor dog is stuck inside his den, which he is loath to foul. When left alone all day with no way to get outside, he must endure the pain of holding large volumes of urine. Add to that the stagnation of urine in his bladder, and bacteria start to grow. Now we have infection. But it doesn't stop at this. Bacterial colonies set the stage for bladder stones and kidney disease.
The simple rule is this: Remember that your dog is a dog.
Worth a read: How long should dogs be left alone?[bookmark_links add_text="Bookmark this Item" delete_text="Delete this Bookmark"]