Every so often, two dogs that attend our Doggie Daycare will meet in our lobby or hallway and start barking and lunging at each other in a not-very-friendly way. The alarmed owners will quickly pull their dogs away from each other. They might then express concern to us about their dog being in the same group with that other dog because they obviously don't like each other.

The owners are surprised to hear that the two dogs get along beautifully, playing very well together.

So why do they lunge and bark so aggressively toward each other? There are two significant differences between meeting in the hallway and in our play groups: the presence of the owner and the leash.

Many dogs become possessive and/or protective of their owners. While Rover and Fido may be best buddies, Fido better stay away from Rover's mom ... or else. This is especially true of guarding breeds and mixed breeds — dogs whose genetics predispose them to be protective of “their own.”

The second, and often the more important factor, is the leash and the effects of physical restraint.

Restraining a dog by tightening up on the leash, applying collar pressure activates the dog's “opposition reflex.” This is a natural response that we all have. It is what a human does, for instance, if someone pushes against her right shoulder. She leans into the pressure in opposition to it. Otherwise she might get pushed over.

When a dog pulls on leash — or the owner pulls back on the leash — it activates the dog's opposition reflex. So when the owner is moving through a crowded area, such as in a hallway moving toward the day-care exit, it is natural and automatic to tighten the leash. The dog then works in opposition to the pressure — pulling harder.

Similarly, if a dog stops moving and the owner tries to pull a dog forward, the dog might well balk, pulling backward in opposition to the pressure. It isn't that the dog is being “stubborn,” which many dog owners assume. Rather, it's simply a reflexive (unconscious) physical reaction.

Just as important as activating the opposition reflex, holding a dog back increases frustration and builds adrenaline, which can cause explosive behavior.

Such aggressive behavior occurs when the dog is on leash, but not when he is off leash, such as in our day-care play groups. That is why, when these same two dogs are running free in day care, unrestricted by a leash and collar, they are best friends. But not when they meet on leash in the hallway or lobby.

This isn't just in a building or related to day care. The same situation might occur at a dog park when two dogs meet on leash or when one dog is barking through a fence. The fence can create “barrier frustration,” similar to the frustration caused by the leash. When a dog is demonstrating fence aggression, he needs to be moved away from the barrier and given an opportunity to settle down before being introduced to another dog.

It is important that when a dog exhibits either leash or barrier aggression at a dog park, the two dogs should not be immediately put together. Once this frustration and adrenaline have built up, the issue doesn't go away simply by removing the leash. It takes time for the dog to calm down — as long as 20 minutes or more.

I offer this as a caveat to dog owners who take their dogs to dog parks where their dogs play off leash. Being mindful of the effects of frustration can help avoid an incident between two otherwise friendly dogs.

Gail Fisher, author of “The Thinking Dog,” runs All Dogs Gym & Inn in Manchester. To suggest a column topic, email gail@alldogsgym.com or write c/o All Dogs Gym & Inn, 505 Sheffield Road, Manchester, NH 03103. You'll find past columns on her website.