Foot fetishes can often be explained by both life experience and biological hardwiring/or mapping in our brains. I agree with ElSeductor that when arousal (initially brought on by kissing/intimate scene in ElSeductor's example) is paired with something such as the foot, our brain has been conditioned to make a connection that the particular stimulus is arousing. This fetish can also be explained by something known as the Homunculus (or Coritcal Humunculus). The Homunculus is located in the parietal lobe in your brain where somatosensory information is sent to be processed, such as touch or even pain. Your body, face, hands, genitals, and feet are actually "mapped" out on this strip of your brain. The key here is that the genitals and feet are close together on the Homunculus. This can cause an individual to experience touch on a foot and experience genital arousal because the touch stimulation message is sent up into the parietal lobe for process and excites both the genital and foot area of the Homunculus, or vice versa.
Interestingly, this Homunculus is used to partly explain phantom limb pain and/or sensations felt in the "limb" by touching completely different parts to the body in individuals who have had amputate limbs.