Disease
Your dog may be suffering from a medical condition such as bladder stones or urinary tract infection, diabetes or kidney disease. If your dog is fully house-trained and they are urinating on your couch, monitor their behavior. Watch for excessive thirst which can indicate that a health problem is present.
Territorial Marking
A dominant dog may urinate on your couch as a way of asserting themselves. When they urinate on your couch, they are marking their territory and will continue to do so as long as they feel that their territory is threatened. Many people believe that spaying or neutering your dog will stop the behavior of territorial marking, however, this is not always the case. Ensure they are receiving plenty of training and remove the scent with an enzymatic cleaner. Consider hiring a behaviourist to assess your dog and make a treatment plan.
Incontinence
Especially as your dog ages, they may begin suffering from incontinence. This is when they do not have the ability to control their urine and will urinate involuntarily. Incontinence will usually occur when your dog is relaxed, usually during sleep. Your veterinarian will be able to diagnose incontinence in your dog. There are various causes and treatment varies depending on what the underlying issue is.
Submissive Behavior or Anxiety
Some dogs that suffer from submissive behavior will urinate on your couch when you enter the room or stand over them. Submissive dogs can be trained to not urinate indoors, but dogs that have been through traumatic experiences may not be able to be retrained to not urinate submissively. These dogs should not be allowed on the furniture. Dogs that suffer from separation anxiety may urinate on the couch because they are stressed by your leaving the home. Again, behaviourist intervention is sensible.
Other Dogs
Sometimes when you introduce a new dog into your home, an established dog will regress in their house-training. Jealousy can also be a motivating factor for urinating on your couch. Dogs that are used to being the only dog may become increasingly jealous of a new dog and will act out by urinating on your couch.