A cat may constantly want to eat because he is bored or has poor nutrition. His polyphagia, or excessive eating, may also be caused by conditions such as diabetes mellitus or roundworm.
Happy, healthy cats don't want to eat constantly. House cats exhibiting polyphagia may have hyperthyroidism or diabetes, both of which interfere with the normal metabolism of food, or other metabolic disorders. Worms, including roundworms (which are communicable to humans), are another common cause of excessive eating. Potential psychological causes include boredom, loneliness and depression. A cat fed on poor quality-food may also overeat in an attempt to satisfy nutritional needs. Correcting overeating may be as simple as buying better-quality food or providing more toys, but it is imperative to have a veterinarian rule out potentially life-threatening medical causes.