Sorry for your loss, it is sad when we lose our grandparents. My grandpa's bday just passed 2 days ago and lost him 17 years ago. I still think of him every year when those dates arrive.
Some people don't realize how precious they are to us until they are gone, it saddens me when I hear the loss of a loved one who meant so much to us.
I have a deep admiration for our elders (since I work in a nursing home facility). I see the loss all the time, but it hurts more when its our own. I lost my grandma 4 years ago and she was a gem to me.
Sorry for your loss. I hope you got to spend time with him before he passed.