Servile Playthings AKA "Pets"
It seems that people want to own servile playthings. A servile plaything is like Caligula's living statues in Gore Vidal's gruesome screenplay; creatures enslaved to serve as ornaments or living toys; like semi-aware portraits or items of furniture. Servile playthings sit where you tell them to sit. They dance when you tell them to dance. They roll over, stand, beg, come, and go by the whims of their master dictator. Servile playthings are not deemed as having souls (Lacking a soul is often used to justify the mistreatment of others and no one can even show the soul exists.). Servile playthings are considered less than, rather then equal to. They have no rights and are often abused to vent the "superior" being's frustrations. Servile playthings lack minds, sentience, intellect, feelings, and moral obligation, thus they may be killed on a whim... or surgically altered to change behavior; to carve away who they are.
I don't believe most people even think of their "pets" as living beings. "Pets" may be loved and treasured companions, but when it comes to the hard choices "Pets" are almost certainly expendable. Human life is always prized more highly; and human laws consider non-human animals as property, with little or no sentimental value. When a lawful settlement is made regarding the loss of a "pet," recovery amounts to the "fair market value of the animal." Which may amount to hundreds, rather then the millions of dollars awarded in similar human litigations. Tell the police that your dog was shot and they will likely shrug... if they didn't shoot your dog themselves in "self-defense" for being too playful. People really think of "pets" as things; as property, bought and sold; as servile playthings.
The "servile plaything" attitude is reflected in the idea that non-human beings should be clipped and tailored... fixed... to better adapt to life as eunuch servants. Beings lacking individuality or consent should have no gender either. Perhaps this is why people generally refer to non-human beings as "it." Non-human beings should have no urges beyond those that are cute and servile. The cropping of gender, tails, and ears helps to carve our "babies" into perfect images of human desire. If non-human beings become too troublesome they may be taken out back and clubbed to death; or for the more "humane," taken to a "shelter" for proper disposal; or taken to the vet and "fixed."
When animal beings are surrendered to shelters, or captured by "dog control," they are lucky if they are saved "the needle" or sometimes carbon monoxide gassing by adoption. When rescued from death row (They usually have three to seven days before execution is carried out), and not yet sexually mutilated, then there is a neuter requirement written into their stay of execution orders. This can apply even if a canine is caught roaming and their owner comes to rescue them; do not let non-humans roam... it's an evil human world out there! Think of adoption as a reprieve; like a last minute call from the governor to stay the execution of a criminal... with an obligation that the reprieved one have it's gender gouged out. Now, what do you suppose the crime was?
Bearing this human attitude in mind, that regard non-humans as mere things, it isn't difficult to see why castration is such a popular method of control. Also, from some of the jokes I've heard people tell, as they take their servile playthings to the vet for castration, I'd say a degree of sadism is frequently involved as well.