You know, it never ceases to amaze me how much of a bond we experience with our pets. From the smallest mouse, to the biggest dog I can never comprehend the bond that us as a human race have with our domestic pets. It's a blessing to have them in your life, and become part of your family, and we weep for them as if they are our blood relatives. They depend on us to do the right thing for them, and to keep them safe and happy. In my humble opinion OP, you did just that and then some. 15 years is a great life for a dog to live, and you did the #1 most un-selfish thing you could do for him. I promise he will be waiting for you on the other side, and you will pick up where you left off. Keep your chin held high, and be blessed just knowing that he was an important part of your family.