The main reason why a dog sniffs another's butt is because he wants to find out more about the dog, it is as simple as that. Dogs are very good at sniffing with great noses, which are much more reliable than that of humans. The anal region of a dog gives out a lot of details about his temperament and character. A dog can learn several new things by sniffing another dog's butt – things like gender, age, present mood, etc. And it can take the place of communication between dogs since they cannot speak like men do.
The act of sniffing other dogs' butts is a very harmless act every dog performs. When dogs smell each other's asses, it is their normal way of saying hi and trying to make friends: it is their most basic effort to start off on a friendly ground. Always allow your dog to participate in this ritual whenever it meets another dog for the first time to prevent fights later. Knowing themselves reduces the frequency of fights in dogs, because they find it easier to maintain a tranquil environment.
By sniffing each other's butt, the male dogs find it easier to predict the exact time the female dog will be in heat, which shows when they will accept mating. If you find your male dog sniffing a female dog's butt, you need to be careful if your dog is not sterilized because it can only indicate interest to mate.
Dogs also use butt sniffing to determine who among the two dogs will dominate; the foundations of their continuous relationship will be by how they interact while sniffing. One of them will always put an end to the butt sniffing before the other dog-this can be done by giving stern warning to the other dog to keep off or by retreating. Whichever one of the dogs that initiates the order and the extent to which the other dog obeys it will go a long way to determine how the relationship between every dog pair will go.
This is due to the fact that the gland contained in the anal region of the dog tell a lot of stories about the dog's personal character, in some cases, playing their cards well is always in the best interest of the dogs in question, and reduce the number of details other dogs can lay hold on by just sniffing their butts. This always happens when one dog sniffs enthusiastically at the butt of another dog, and the dog being sniffed at clamps its tail down to conceal their scent very well, and making attempts to move away from the sniffing dog.