There are a few well-known, well-established, and well-studied cognitive theories that explain why people perceive things like this. Namely (in order of underlying to specific):

Apophenia - http://en.wikipedia.org/wiki/Apophenia
- Apophenia is the experience of seeing patterns or connections in random or meaningless data.

Pareidolia - http://en.wikipedia.org/wiki/Pareidolia
- psychological phenomenon involving a vague and random stimulus being perceived as significant. Common examples include seeing images of animals or faces in clouds, the man in the moon, and hearing hidden messages on records played in reverse.

and of course:

Confirmation bias - http://en.wikipedia.org/wiki/Confirmation_bias
- confirmation bias is a tendency to search for or interpret new information in a way that confirms one's preconceptions and to avoid information and interpretations which contradict prior beliefs.

Put all of those together, a dash of religiously-motivated ignorance and.....voilà! jesus/God/Mary faces in things....

*golf clap*

(of course, the religious will look at this and say, "that's just scientific mumbo jumbo..." and thereby demonstrate Confirmation Bias...