I would say that there is not only one reason someone avoids eye contact. Being shy, nervous, not having many social skills, can all lead to this behavior. I have looked away from people that are very domineering and glare at me, but I wasn't being dishonest. I just didn't feel like staring back and forth.
I think that context is also important. If it is a discussion with a romantic partner, if a sensitive topic was touched on, or a childhood issue that makes someone very uncomfortable and is difficult to discuss, they may avert their eyes.
maybe the eyes makes you feel uncomfortable, or they are too unconvincing, or having trouble following what they are telling you, maybe to many distractions the other direction
I seldom look into a person's eyes. This is not a conscious decision nor a reaction through nervousness at the individual's presence. My avoidance of eye contact is due to poor social skills. The reasons will be varied with others.
I rely on, process and accept what I hear primarily. I used to get punished as a kid for not making eye contact with adults but I never refused contact or communication with one. I'm really creative and growing up my preferred way to share was through my schematics and designs. So sometimes there is a valid or undiagnosed personality issue. I focus on the relationships between people, motives as opposed to an individual. Also, I prefer to have my body in motion while communicating. I readily offer eye contact on an individual basis but it's contextual.
I tend to gaze at people for no real conscious reason that I've ever been able to fathom (due, up to now, to an acute lack of interest or self awareness). Some find it probing....some find it intimidating....some 'fight back'.......I wonder....(!). I agree with Nero. Context matters. I know a very shy person that maintains eye contact as a mask for example.
I was actually just noticing this today, but have no idea why people do it. I can't stand to look someone in the eyes even when I make a conscious decision of it. If I do it seems to drown out everything they say, like I'm focusing on their eyes too much to hear what they're saying. Does this make any sense? What's up with eye contact that makes it so awkward to me? I'm social and truthful for the most part, and not autistic.....I don't think.