What is the "greatest treasure of all" in the first few chapters of "Where the Red Fern Grows"?

From the text:

Although the old hound had no way of knowing it, he had stirred memories, and what priceless treasures they were. Memories of my boyhood days, an old K. C. Baking Powder can, and two little red hounds. Memories of a wonderful love, unselfish devotion, and death in its saddest form.