Why did the narrator let the dog loose?

The narrator didn't want to pen the dog in. He would have kept him had the dog wanted to stay, and yet, he knew that the dog had somewhere he was trying to go.

I didn’t have to let him go. I could have kept him in my back yard, but to pen up a dog like that is a sin. It would have broken his heart. The will to live would have slowly left his body.

I figured something drastic must have happened in his life, as it is very unusual for a hound to be traveling all alone. Perhaps he had been stolen, or maybe he had been sold for some much-needed money. Whatever it was that had interrupted his life, he was trying to straighten it out. He was going home to the master he loved, and with the help of God, he would make it.