A Thai man has made it his life's mission to help people through their grief with animals. (https://www.bbc.com/news/topics/clm1wxp533pt/animals)
There's no doubt that when pets are lost or die, there are those who mourn their loss.But it raises the question of whether or not that grief goes two ways? What do you think - have you ever heard of an animal missing its human or animal friend? (If you do, then you can point others to examples of that on the facebook page of International Animal Law).
If you're of the view that "they're just animals" then maybe its not a consideration for you about what animals might feel. Indeed, though law only draws the line at "pain" or "distress" that is deemed "unnecessary or unreasonable", imagine how society's conscience, awareness and actions might shift if it became a legal standard to take responsibility not just for their negative experiences (e.g. pain and distress) but also for providing animals with opportunities for comfort, interest, and pleasure.
Now how that might apply in terms of loss of a friend (human or animal) is open for debate - but consider other kinds of "loss". How might farm husbandry systems shift to accommodate the separation of a cow from its calf, for example? Or companion animals left alone for extended hours in a backyard? What other examples can you think of where the standards of animal care might require adjustment if law recognised that animals experience more than simply "pain" or "distress" and applied a responsibility on people to provide animals with "opportunities" for "comfort, interest and pleasure"?