Bad parent

I took Frankie back to the vet for x-rays and more tests - nothing obviously wrong. The doctor said two things are fairly common in cats - lymphosarcoma and inflammatory bowel disease - but both require endoscopy to draw definite conclusions and neither are curable. I'm starting to feel like I've done something wrong to make both of my boys sick at once. They eat premium food, get plenty of love, fun, playtime and exercise. What did I miss?