His heart will have difficulty pumping blood through his system. His body will be absorbing a high amount of fat, which will also effect how his heart distributes blood and nutrients through his system. His immune system may not function properly. His kidney's may be unable to excrete poisons effectively out of his system, which means he can develop all sorts of aliments - including diabetes. If he develops diabetes his ability to heal, his circulation, and his eye sight may all be at risk.
Obesity can also cause lung, liver and heart diseases.
One of the most common reasons is Hormonal changes. Ovaries and testicles both contain a food inhibitory hormone. When the ovaries and testicles are removed and or the dog ages, this hormone is reduced in the dog's system, which causes weight gain. But that time never comes, because the dog is continually fed excess calories. Feel your pet's ribs every two weeks.