One of the factors which makes or breaks a photograph is the eye contact.
A photograph with strong eye-contact cuts straight into your soul.
If you have ever sat on the bus, on the subway, or in any public setting— you might have sensed the feeling of someone staring at you from your periphery. You feel nervous, tense, and awkward. You might look over your shoulder, and see that person looking straight into your eyes. The second you make eye contact, you will both instantly feel awkward— and either the other person, or you, or both of you— will look away.
Modern humans have a hard time making eye-contact with others. This is because when we make direct eye-contact with another human, it is a threat. Both to us and to the other person.
The other person might be wondering, “Why is that stranger looking directly at me? Are they going to attack me, steal something from me, or are they judging me? Do I look weird or strange today? What is that person’s problem?”
When you are staring at someone else, you actually are secretly judging them. You are trying to get a read on them, and trying to figure out what kind of person they are, by the clothes they wear, by the way they carry themselves, as well as their body language.
When you have a photograph in which the subject is looking directly at you (the viewer), we get that same sensation. We feel nervous and excited when someone is making direct eye contact with us.
As humans, we can read a lot about someone else through eye contact. We can get a sense whether they are friendly, an enemy, or feeling sad or lonely. We can empathize with another human being through the eyes.
So in your photography, see how you can incorporate eye-contact (or lack of eye contact) to make stronger images.
If you see someone interesting on the streets, keep taking photos of them, until they sense your presence, and then they turn around and look at you.
Once they make eye contact with you, then start taking photos. That photo with eye contact may be stronger than the photo without eye contact.
However there are lots of portraits in which the viewer isn’t looking at the photographer which are strong. My suggestion is to study the work of portrait painters from the past.
Also whenever possible, try to take photos of your subject with and without eye contact. Then afterwards, you can sit and look at your photos and figure out which version works better for you.
But for me, I will always prefer eye-contact. To me, photos with eye-contact are the ones that are personally-meaningful to me, memorable, emotional, soulful, and burn themselves into my mind.
Experiment with both approaches; and find out what works best for you.