There has been scientific evidence supporting the theory that as humans, our brains are programmed to instinctively recognize faces. Perhaps initially as a defensive mechanism to detect friend from foe. But one can only speculate.
“While the human tendency to see faces in other objects is rooted in neural architecture, the large number of actual faces we see every day may also be partly responsible for the Nun Bun phenomenon", said Takeo Watanabe, a neuroscientist at Boston University. His studies of learning processes show that after the brain is bombarded with a stimulus, it continues to perceive that stimulus even when it is not present” (Svoboda).
“Our brain is wired to find meaning. Our aptitude to identify structure and order around us, combined with our superior talent for face detection, can lead to spectacular cases of pareidolia, with significant effects in society and in culture” (MARTINEZ-CONDE). For each of us has a very unique and random set of stimulus determined by the people we meet, the choices we make, our beliefs, memories, etc.
Martinez-Conde, Susana, and Stephen L. Macknik.”A Faithful Resemblance." Scientific American Mind (2012): 19-21. Academic Search Complete. Web. 6 Nov. 2012.
Svoboda, Elizabeth. "Faces, Faces Everywhere." New York Times. 13 2007: n. page. Web. 6 Nov. 2012.<http://www.nytimes.com/2007/02/13/health/psychology/13face.html?pagewanted=1&_r=0>.