Why do people look in the face or eyes while communicating? While communicating, people tend to direct their gazes to the face (particularly eyes) of people. Why don't rather we look at their neck or just their chest or even their hair while communicating with people? What are we looking for? 

A sign, a hint about that person's thoughts. Obviously that is the case ,but on the other hand, I think there is something more about that. Take those movie scenes in which people's head cut off their shoulder and they are still alive. I think the whole body doesn't seem human to me, whereas the head seems definitely so. Is it just a stable reference point that we've picked after centuries, or is it a native property specialized for a certain purpose of human kind?
Eye contact is one of the principal cues humans use to evaluate where other people direct their attention. A special issue on the use of eye tracking in the Infancy journal and other studies reported that infants' eyes are useful measures of attention over a range of task domains including object perception (Johnson, Slemmer, &amp; Amso, 2004), faces (Hunnis &amp; Geuze, 2004), and categorization (McMurray &amp; Aslin, 2004). 

This relationship between eyes and attention begins developing as early as within the first couple of neonatal weeks (Lohaus, Keller and Voelker, 2001). For the same reason, eye- and gaze tracking are central measures in many areas of the cognitive sciences, including developmental (neuro)psychology, perception and working memory. 

Because gaze is such a good general indicator of attention, mutual eye contact between myself and someone indicates that we are both attending to one another, if not always, then at least often enough to depend on.

References


Lohaus, A., Keller, H., &amp; Voelker, S. (2001). Relationships between eye contact, maternal sensitivity, and infant crying. International Journal of Behavioral Development, 25(6), 542-548.
Hunnis , S. and Geuze , R. H. 2004. Developmental changes in visual scanning of dynamic faces and abstract stimuli in infants: A longitudinal study. Infancy, 6(2): 231–255. 
Johnson, S. P., Slemmer, J. A., &amp; Amso, D. (2004). Where Infants Look Determines How They See: Eye Movements and Object Perception Performance in 3‐Month‐Olds. Infancy, 6(2), 185-201.
McMurray , B. and Aslin , R. N. 2004. Anticipatory eye movements reveal infants' auditory and visual categories. Infancy, 6(2): 203–229.