Marty insists Judd include it because he wants to ensure that Judd understands the commitment he's making to taking care of Shiloh, not just for a short period but for the entirety of the dog's life.
The phrase "forever" is a powerful symbol of the deep bond between humans and animals, and Marty wants Judd to recognize the weight of that responsibility. This highlights the theme of loyalty and commitment in the book, both between humans and animals and within families.