Dave, my heart aches for you.
The thing about Man's Best Friend is that they judge you soley by the way you treat them, your outward and inward goodness and your willingness to let them live in the moment. In many ways, our dogs / friends can take us to that wonderful place no matter what our day, week or mood tends to be, and they only want one thing - happiness.
Perhaps this is why we grow so close to them. They judge us not by how much money we make, what shirt we're wearing, whether or not our grammar is perfect, or our breath smells bad. They simply do not care about things that do not matter in the long run; they can only care about the here and now and that which gives them a reason to love us.
Perhaps this is why so many of us gravitate towards these find friends. Perhaps it's because so much of that gets in the way of our human-to-human relationships. Your dog does not care what car you drive, what shirt you wear or whether or not you like one Philly cheesesteak sandwich versus another.
They crave what we all crave, love.
And I am certain Dave, that this is exactly what you got from your wonderful friend over many years.
That is replaceable, and reason enough for your sadness.
Joel