To forgive someone means to let go of it, and be able to live in the present moment, and not dwelling in something that happend in the past.
But it does not seems like human beings are able to forgive anymore? Why is this so in your understanding?
People are layered, and those layers hide our own inner workings. For a long time I confused low self esteem as a generous nature. I had the appearance of a good nature and knew what it was like to behave like I did. Forgiveness did not seem difficult, much like death does not seem real to younger men or doing a favor for your son doesn't seem difficult. There are actions we take which appear generically forgiving or good which don't reflect an inner practice and commitment. These can deceive us into thinking we don't need improvement.
Another reason people have trouble forgiving, besides self deception, is that it feels good to be angry and right. Feeling good can be very important to someone who is not happy or who is in the habit of the highs that anger can bring. Anger can make you feel good, not just bad.
Shame is another reason people can't forgive. All kinds of things cause people shame both deserved and undeserved. We are none of us truly independent creatures and need to see ourselves reflected in others. If our experiences bring shame, then they bring rejection with them. Our emotional intelligence and ability to differentiate comes into play, but a lot of people do not have a strong ability to separate false shame from healthy shame.