Is there a reason for?

Along with the information they get by sniffing the behind, it is an absolute 100% non threatening way to greet another dog. A face to face meeting may be seen as a challenge, and encourage them to square off.

Unlike humans, dogs can't just look at each other and say male or female so they have to smell the genitals.Its also a way of saying HI! this is normal, be more concerned if you have a dog that don't do this and just stares at other dogs.

That is absolutely normal for dogs. It is there way of greeting other dogs and knowing their gender, their genetics, their health and even age. Smelling their rear helps a dog better understand all kinds of information in not just dogs but in other animals and even us humans.

The dog's anal glands produce a scent that is specific to the individual dog- kind of like fingerprints.

This is why dogs are also interested in sniffing other dog's poo- because when a dog poos the dog's scent also comes out of the anal gland and attaches itself to the poo- like a calling card.

When a dog smells another dog's butt they are sniffing the anal glands to identify the dog.

You have to remember that dogs aren't like people. They rely on their sence if sight least, unlike humans who use sight as the main way we recognise people and places.

Dogs have a sense of smell that makes ours look miniscule- and they use this fantastic ability to identify other dogs, people, places, sniff out food, water, etc.

It is just a way for dogs to say hey and find out about the other dog. It may be a little strange because humans don't do that (well I hope).

Yes. There's a gland there that tells information humans ask each other when they first meet, such as "Who are you? How old are you? What did you have to eat today?" Etc.

that's how dogs get their information from other dogs, and other animals as well. they can't go up to another dog and ask him/her questions like "what did you eat today, where did you go, who did you see?" so sniffing each other is their way of gaining all that information.

Yes, there is. It's their way of meeting each other. Dogs don't look at other dogs to know who they are. They know each other by scent. It is necessary and normal for them to sniff each other.

It's like a handshake!