From yogaslate.com :
he sense of smell may seem an odd choice as a focus for meditation. But consider the power of the olfactory system in our lives: Vivid memories can be summoned instantaneously just by taking in a particular scent, and those memories often come with a wave of associated emotions. For instance, the smell of bread in a bakery reminds someone of her beloved grandmother, or a whiff of ocean breeze takes another person back to his first fishing trip with dad.
A New York Times article refers to the nose as “an emotional time machine,” and describes research demonstrating just how significant scent is to our species, and how memories triggered by scent typically take us further back in our lives (before age 10) than those summoned by our other senses.