Many things I could say about this @jackmiller... part of what happens to "old sheepdogs" is perhaps a reflection of how many societies really don't value aging and wisdom; in spite of all their contributions to the world, the elderly (in most western culture) are sent to rot in nursing homes. The old sheepdog doesn't get to live out his days in comfort on a cushy bed on the porch, watching the young pups in training.
For the most part, the sheep lack the awareness to know that something was done "for" them to protect them... and so the gratitude the sheepdog gets may only amount to the knowing that sheep didn't die a horrible painful depth on his/her watch.
Perhaps what this points to is that being a sheepdog requires a somewhat idealistic nature. Because if you just distill it down to direct rewards, there is less point; less purpose.
I'm sure that the sheepdog even in his/her last days would still take the bullet for any lamb out there. No matter how bitter it may be.