Grief & loss regarding the death of a pet is often misunderstood and harshly judged by people. "Disenfranchised grief" is a term used to describe the type of grief that is not accepted or understood by society.
This can result in a grieving pet owner feeling they have to deny or "stuff down" their grief, which compounds their grief and can lead to physical and emotional difficulties.
I believe pets are gifts from Heaven, given to us to reflect the most pure, unselfish, unconditional, joyful love that we will ever experience here on earth.
In my experience, many people have closer and more loving relationships with their pets than they have ever experienced with a person. Considering this, it is only natural their grief will be significant.