Aside from animals not having rights: 

The most common reason dogs and cats are abused is neglect, often stemming from owner ignorance, financial hardship, or apathy. For intentional abuse, perpetrators usually act out of a need for power and control, displaced aggression, or due to severe mental health issues.

Reasons for neglect (passive abuse)

Neglect is the most widespread form of animal abuse and is often the result of an owner’s inability or unwillingness to provide basic care.

• Lack of awareness: Some pet owners are unaware of their pets’ fundamental needs, such as proper nutrition, socialization, and suitable living conditions. This can result in unintentional neglect.
• Financial strain: Economic hardship is a significant driver of neglect, particularly amid rising pet food and veterinary care costs. Owners with limited resources may not be able to afford necessary medical treatments or even proper food.
• Changed life circumstances: Significant life changes can strain an owner’s ability to provide care. These can include job loss, divorce, a move to a new home that doesn’t allow pets, the birth of a baby, or the owner’s illness or death.
• Unwanted litters: Pet overpopulation resulting from unspayed or unneutered animals can lead to neglect and abandonment when owners become overwhelmed and unable to care for all their animals properly.
• Boredom and indifference: In some cases, pets acquired on impulse are later viewed as a burden. The owner may lose interest in the animal, leading to boredom and inadequate care.

Reasons for intentional (active) abuse.

Intentional abuse involves deliberately inflicting pain and harm. These acts are often correlated with severe underlying issues in the abuser.

• Mental health issues: Psychological disorders, such as antisocial personality disorder and conduct disorder, are strongly linked with animal abuse. A lack of empathy, high impulsivity, and low frustration tolerance are common traits among abusers.
• The link to domestic violence: Animal abuse is often a component of domestic violence. An abuser may harm or threaten a pet to intimidate, manipulate, and exert power and control over a human victim.
• Displaced aggression: A person who has been a victim of violence themselves may take out their hostility and aggression on a more vulnerable creature, such as a pet.
• Power and control: Abusing animals can give individuals a sense of power and dominance, especially if they feel powerless in other areas of their lives. This behavior can manifest as cruelty or as a means to establish dominance, such as in illegal dogfighting.
• Cruelty as a precursor to other violence: In some of the most serious cases, intentional abuse is a strong indicator of escalating violence. Many serial killers have a history of abusing animals as children.
• For entertainment or “amusement”: Some people harm animals for sadistic reasons or to impress others with their capacity for violence. Children, in particular, may harm animals out of curiosity or peer pressure.