Chances are you are feeding your cat junk food.  I'm sure it's not intentional.  Maybe you don't know that the dry foods with the fish shapes and bright colors are like cocaine to your cat.  A few bites of his cat kibble will make his blood sugar jump sky high, because he does not have the enzymes necessary to digest starch.  It's BAD for him, and it causes all kinds of illnesses, long term.