I felt much the same when my own grandfather passed. He had been sick for 5 years, and a dirty bastard his whole life. I was mostly concerned for my grandma, who had taken care of him up to his death. She did this while sacrificing her own health and happiness. When he died I was relieved for her.
At the funeral, my uncle delivered the eulogy in which he praised my grandmother for being such a devoted wife and caretaker. This was a very fitting tribute, because if he had tried to say nice things about my grandfather, the speech would have been much shorter. The only tears shed were out of relief.
Count me in with the people who didn’t grieve profusely when I lost my grandparents. My last grandmother died this winter, and while she was a good lady, I had almost no connection to her (I can’t honestly remember ever having been hugged by her), and had only seen her a couple of times in the last 12 years. I felt badly that I wasn’t grieving more, too (it worries me if I don’t seem to feel emotions as strongly as other people), but her passing has made almost no difference in my life, I’m sorry to say. Maybe that’s what makes up most of grief; the day-to-day things that change when you lose a loved one.