The dog brain is a large part known as the olfactory bulb, dedicated to olfaction or smell.
Because it is such a large portion of their brain, dogs rely on their sense of smell for many purposes including communication with other dogs. With their incredible sense of smell, dogs read, learn, and navigate the world around them. For this reason, dogs tend to sniff a lot, especially when they are outside of their normal environment.
Because of their smelling abilities, humans have begun training dogs to use their sense of smell to sniff out illegal substances and aid the rescue team.