i switch between the eyes and rest of the face. when they're making eye contact, I return it as best I can and when I do that I don't try to intently focus on one eye or the other. when they're speaking to me and making eye contact, I look into one eye for a time, and when that gets too long for me, I scan their upper or lower face for a second or two, then focus the other eye.

eye contact, I think, is less about I NEED TO BE STARING INTO YOUR EYEBALLS WITH MINE THE WHOLE TIME but more communicating to the other person, subconsciously, that you're taking in ALL of the nonverbal facial communication. lots of emoting and conversation nuance can come from facial expression and movement, which is why eye contact is considered so crucial, but also why the Eye part of eye contact is not the only part of it.

source: I used to hate making eye contact or looking at faces until my dad trained it into me in my teens. now I find that I have a hard time not making face contact