Ever heard of the phrase "the eyes are windows to the soul?" Yeah? You know why? Cause it's true.
Let me start off with a scientific explanation.
Your sight contains the most information out of all your other senses, and your sight information goes to the brain faster than any other sensory input (look at the location of the eyes, you could also argue the ears send information just as fast but that is off topic). Sight information is very complex to the brain - your brain has to judge distance, take in everything present in your view, bring up old memories and experience, calculate imminent danger if present, and make snap judgements all in miliseconds. Everything you see is because of wavelengths - specifically the spectrum of visible light so your brain has to process many different dimensions of wavelengths (and quite possibly beyond visible light) in order to understand what is going on. I hope I have made this clear.
Now, why are people's eyes so useful to us should we want to understand them? Here's an example: you can immediately tell if a person is lying or not just by looking at his/her eyes when you ask a question. HOWEVER, this requires skill. You, yourself, must be honest, caring, and open minded to do this. Here is a hypothetical situation:
Open up your eyes very widely (so your eyes can absorb as much information as possible) and ask them "Are you lying to me?" Pay attention to the flickers, muscle contractions, and pupil dialations. Their brains are processing the information very rapidly, BUT it is confused. A lyer will be more tense and slight movements will be more jerky (they want to escape); a truth speaker will be more clumsy or calm depending on how surprised they are (they might be a little confused at first, but you should also take into account voice tone and other facial features as well as body language).
Now, let me make a few things clear. This is NOT to be abused. You are not to ask random people around you: "Hey, did you steal my wallet?" when your wallet is clearly in your pocket. Why shouldn't you do this? Because looking into people's souls through their eyes is a skill. If you constantly constantly hit people with your tennis racket instead of the ball, will you ever be a good player? No, and you'll even screw up your own learning process.
Also, like every other skill, you need to have certain qualifications. As I said earlier, you, yourself, must be an honest, loving, and open minded individual. This will backfire on you if you are not this sort of person because your own brain will be confused should you attempt this. If you don't bring a tennis racket to tennis, can you play tennis?
You can also use this skill to judge people's current emotional and mental state, but all of this is very, very fine and delicate information that is almost subconscious. But practice makes perfect, so be a good person today.