Why Do Dogs Sniff Around?

They are marking their territory. They will either smell another dog's
urine and urinate over that or, they will mark their own territory such
as the backyard/frontyard, etc. It tells them that that
is their territory. If one dog, wolf, ... Sees another dog mark its
territory onto theirs, it will usually start a fight. Think about this
example to help you understand if you get a glass of milk and you take
a drink, that cup is your territory until you are done. Lets say your
little brother or sister comes into the room and takes a drink out of
your cup. You get mad because that cup is your territory. It is the
same thing with dogs, its normal.www.sponkit.com

The sense of smell to dogs is like our eyesight. In fact my dog is going blind and sniffs his way back into the house. They seem also to enjoy the variety of smells in the air as witnesses by anyone who has a 'car dog', that enjoys sticking his snout out the window while driving. The sniffing tells them and their body where to go to the bathroom. It tells them who has been here and where. It tells them where you have been, and who you are, and they can smell the smell of FEAR on people which is why they chase people who are afraid of them. A dogs snout is to them, lik our eyes. Dogs don't see really well to begin with and many develope cataracts as they age. They recognize things by smell, and their hearing is more powerful than ours which is why they make good watchdogs.

They are marking their territory.

I actually have my own question......why does my dog always sniff my hair?....He is a beagle