Complications of diabetes. He had been on insulin for a few months about a year prior and had taken a year off being diabetic. On my dad's birthday, he started producing ketones and had kidney renal failure. I had to put him to sleep.
I'm so sorry to hear that. I really do like my cats but I've learned not to get too attached. Sooner or later they find out what the highway is all about. I've lost so many because of it.
Losing a pet is so hard it is curious why we let ourselves fall for them as we do! I love my little dog, and will not be able to function for a long while when I loose him.