my golden died 2 years ago.  she was 13, and perfectly healthy and active until one day she just lay down and literally fell apart from the inside.  it was pretty gross and horribly sad, her face swelled up from internal bleeding.  and she lay there for 2 days, unable to move.  I just hugged her and then finally had to say goodbye on day 3, my mom had her put down because it was too awful to see her lying there suffering.
my new dog is the shit though, hes seriously the worlds best dog.