My mom's German Shepherd died suddenly over last Thanksgiving. She was 9. We still don't know what happened to her, we think something in the backyard had bit her because one day she limped inside and her left hind leg was swollen.
We took her to the vet 3 times in two weeks, and they were baffled. They ran a blood test and everything, but couldn't tell us what was wrong with her. It was strange - one day she was fine, and the next she was really lethargic and just laid in the same spot all day. She stopped eating, and didn't even want to go outside anymore. Then a few days later she was like her usual self.
They think it was a rare cancer that affects only German Shepherds, but I thought it was too convenient for them to just blame it on that. It was so sad, she was a total sweetheart of a dog, it shouldn't have happened that way. She was a member of the family, and when left us it was really hard on everyone. I still think I could have stepped in and pushed the vet to do more testing, or maybe taken her to another vet. I don't know.
It definitely sucks, especially when you become so attached to a family pet. I remember when she died, I called my mom to ask her how she was doing and she said she was just laying down behind the couch like she did. A few hours later I felt like something was really wrong and I called again, she had just died.
It sucks when dogs die. Sorry to hear of your loss, it hurts real bad but the pain fades in time.