Why is my dog attacking my other dog all of a sudden?

Watching your dogs, who once curled up together like old friends, suddenly snarling and lunging at each other feels like a gut punch. One moment they’re sharing toys; the next, it’s a tense standoff. This unexpected aggression leaves you scrambling for answers—and it’s more common than you’d think. Behind that sudden outburst lies a complex mix of physical, emotional, and environmental triggers, and understanding them is the first step to mending the rift.

Health issues often lurk beneath the surface of sudden aggression. A toothache, ear infection, or even arthritis can make your dog lash out when touched or jostled by a playmate. Imagine feeling constant pain and having someone bump into you—you’d snap too. If the aggression seems directed at the slightest interaction, schedule a vet visit immediately. In many regions, pet owners are legally obligated to address medical conditions that could endanger other animals, and early diagnosis can prevent minor discomfort from escalating into dangerous behavior.

Resource guarding transforms your sweet pup into a fierce defender. Whether it’s a coveted chew toy, a bowl of food, or even your lap, dogs instinctively protect what they perceive as valuable. New arrivals in the household, like a puppy stealing attention or a new treat dispenser, can intensify this behavior. It’s crucial to establish clear boundaries around resources and teach “drop it” or “leave it” commands before tensions boil over. Ignoring resource guarding not only risks injuries but may also violate local animal control ordinances if disputes lead to bites.

Social dynamics shift over time, especially as dogs age. A once-tolerant older dog might grow impatient with a hyperactive youngster’s rough play. Hormonal changes can also play a role—unneutered males may become territorial, while intact females in heat can trigger aggression in males. Spaying or neutering is often recommended not just for population control but also to reduce aggressive tendencies. Many shelters and rescues promote responsible pet ownership by offering low-cost spay/neuter programs, which can ease household tensions and keep you compliant with local pet regulations.

Environmental stressors act as invisible agitators. Construction noise, a new neighbor’s barking dog, or even a change in daily routine can fray your dog’s nerves. When dogs feel anxious, they may redirect that pent-up energy onto a convenient target—usually a housemate. Notice if the aggression spikes during thunderstorms or after rearranging furniture. Creating a calm space with a cozy crate, white noise, or pheromone diffusers can help soothe frayed nerves and prevent outbursts.

Sometimes, the root cause traces back to a traumatic event. A scary encounter with another dog on a walk, an accidental injury during play, or even separation anxiety can resurface as aggression. Dogs process fear differently than humans, and what seems minor to you could leave lasting emotional scars. Gentle counterconditioning, like rewarding calm behavior around the trigger, may help rebuild trust over time. But if aggression persists, consulting a certified dog behaviorist is crucial—both for your pets’ safety and to avoid potential legal issues if aggressive incidents occur in public spaces.

Addressing sudden dog-on-dog aggression requires patience and consistency. Separate the dogs when unsupervised, even if it breaks your heart to see them apart. Gradually reintroduce them using positive reinforcement techniques, like offering treats for calm interactions. Remember, it’s not about punishing the behavior but teaching new, more appropriate ways to interact. With time, love, and sometimes professional help, your dogs can learn to coexist peacefully again.