In general dogs get diabetes due to many different reasons. It is happening more often now than it ever has. The main reason is spoiling the dog with too many treats or designer dog foods.
Some reasons are...
~ Overweight dog that does not get enough exercise for the amount of food/treats the dog is eating.
~ Brand of Dog Food (even the expensive designer dog foods can be very bad for your dog) - Some foods are just too high in Carbs or contain sugar in them. All semi-soft dog foods have some form of sugar in them this is what helps them stay semi-soft.
~ Breed or Genetics of Dog
~ Other illness
When your dog becomes diabetic, the dogs pancreas is not producing enough insulin (insulin is a hormone). The hormone "Insulin" helps regulate blood glucose (also known as blood sugar). Blood glucose is not regulated when there is no insulin or there is low insulin levels in the body. The liver then tries it's best to filter out the high blood glucose but it works overtime trying to get rid of the high blood glucose. This causes your dog to become dehydrated and that is why your dog drinks so much water and then pees so much. If you have noticed the dog urine is very sticky if it dries on the floor. It is like someone spilled sugar water on the floor. The body naturally produces blood glucose, but what a dog eats also increases the normal level of blood glucose. Foods that are high in fat, carbs and sugar increase blood glucose.
So if you are reading this and you have a dog that is drinking excessive amounts of water and peeing in the house (or needing to go outdoors) very often and you have not got into the vet yet with your dog. You should go buy your dog some floor pee pads, some Hill's Science Diet "Light" dog food (either can or dry or both) and something that is low fat and low sodium or (no salt) to use as a topper to get the dog to eat the new dog food. You do not want to mix the new dog food with your old dog food.
Toppers can be Low Fat Cottage Cheese, Low Fat Plain Yogurt, Canned Pumpkin (No spice type, Libby's makes both and if it is spiced it will say on the front label), Low Fat/Sodium Chicken Broth. You want at least one of these on hand if your dog refuses to eat the Hill's Science Diet "Light" dog food. Feed your dog only the amount that is recommended on the dog food label based on what weight your dog should weigh. Only use a "topper" to mix in with the food if the dog will not eat it without the topper. 1 heaping tablespoon of a topper per each cup of dog food used should be enough of the topper, but slightly more can be used. Try different toppers (most dogs love canned pumpkin). Mix the topper in well with the dog food. Put the dog on a every 12 hour feeding schedule (remember to read the food label and split the recommended daily amount into 2 feedings).
Do this until you can get your dog in to the vet. Even if your dog stops drinking excessive amounts of water and peeing a lot, you must take the dog to the vet as it will have to be checked out and put on insulin if it is found to be diabetic. You cannot cure or fully treat diabetes with just a diet change. The purpose of this is some dogs respond well enough to this diet change to stop the peeing in the house. It can slow down or stop the excessive thirst and constant peeing until you can get your dog into the vet's office.
Never withhold water or food from your dog. The Hill's Science Diet "Light" dog food is almost exactly the same formula as what most vet's put diabetic dogs on. You can go to the Hill's Science Diet website and look at the dog food labels. Many vet's sell a prescription dog food called Hill's Science Diet W/D (W/D stands for Weight/Diabetic) and it is costly. (about 2 times higher than the Hill's Science Diet "Light"). Either one is fine to put your dog on, it will help greatly in getting your dog's thirst and peeing down. Some vets will put your diabetic dog on the Hill's W/D (or Light) before the dog is put on insulin. The Hill's W/D (or Light) helps slow the body down with creating blood glucose at a slower rate. It works and it may give you some rest and help save your floors from a dog that most likely is diabetic.
(At the time of writing this my vet charges $24.50 for a 8lb bag of dry Prescription Hill's Science Diet W/D ~vs~ $24.50 for a 17lb bag of dry Hill's Science Diet "Light" from my local farm supply store. They also carry the dry "Light" in 5lb bags for $11.00 - The dry "Light" also comes in small bites kibble size and medium kibble size, the W/D only comes in medium kibble size. Small dogs like the "small bites Light" size. The canned "Light" is also about 1/2 the price of the canned Prescription W/D. Some vets use Prescription Royal Canin or Prescription Purina Pro. I don't know enough about them to suggest a non-prescription version of them)
Again please get your dog to the vet ASAP, if you think it has diabetes or is sick. The diet change suggestion is here to help those that cannot get to a vet ASAP but need help in getting the dog some relief from the excessive drinking and peeing and maybe help you get some rest and save your floors in your home. Diet will take care of 1/3 of the diabetic issue, Insulin is the other 1/3 and schedule, care, checking BG (Blood Glucose) levels and the correct amount of exercise is the other 1/3.
My dog was put on Prescription Hill's Science Diet W/D for 3 weeks before he was put on insulin. (started 12/28/11) He also got a 10 days supply of antibiotics in the event he had a kidney or bladder infection. He stopped eating the W/D after about 3 days and I had to introduce a topper to it so that he would eat it. My dog was pretty sick and was drinking and peeing a lot before he saw the vet on 12/28/11. After he was put on the diet the drinking and peeing reduced dramatically. It got his BG down enough to stop the the excessive drinking and peeing but of course it was not enough to stop him from needing insulin. He started insulin on 1/20/12 (15 days after his 6th birthday).