Direct eye contact is said to be a polite way in personal grooming. This is because it is not good manner to look somewhere else when a person talks to you.
However, most people are not comfortable with long period of direct eye contact.
The reason is simple. They are afraid that they will lose themselves. Why? This is because the longer you look into the eyes of another person (or even animals), the more you are absorbed into it). Yes, this is hypnosis. When you are absorbed in it, what will happen is you are actually stepping back from your conscious mind into your subconscious mind. You may have difficulty thinking straight.