Direct gaze - that is, eye contact with another person - causes the subcortical system to overload, and the parts of the brain that deal with arousal and calming are failing to strike a balance. The end result is that direct eye contact triggers a physiological response which makes it physically uncomfortable to maintain eye contact.
there’s also the base physiological fact that there are two eyes, and you can’t look at both at the same time. This results in either choosing one eye (awkward) of flickering between the (eye-strain tastic). Both are terrible options.
Instead I look over the whole face so I can attempt (really badly) to parse feeling & intention. This often results in making comments like ‘You have amazing ears’ or ‘that lipstick is perfect’ in the middle of otherwise entirely mundane conversations.