I love old dogs.  More often than not, I pause to pat them, and they look up at me with old eyes, full of appreciation and resignation.
Dogs live for their walks. On the beach, the old dogs walk slowly, lagging behind their owners, their heads down, stiff-limbed moving with dogged determination. Even though it takes a huge effort, they soldier on. 
Old dogs look kind. In general, they endure their declining years with good grace. They cause a welling of tenderness.