Pheromones? Just like how animals secrete special hormones to attract other animals, humans are subconciously attracted to the hormones of the opposite sex, too.
And now for the d21lewis answer: If I can smell your pussy, there's a pretty good chance that your panties are already off and my face is already covered in pussy juice. OF COURSE I'm going to......hey, this turned out to be a lot more vulgar than I originally intended. I'm sorry.
*walks away with head hanging down*