How do you turn over a new leaf? How do you transition from being someone that you simply are to being someone that you aspire to be? How do grow as a person in such a way that there is an external change as well as an internal one?
For all the motivational material out there that is dedicated to weight loss, skills acquisition, goal seeking, and so forth, those things seem relatively easy to me. That is, they seem easy to me compared to the task of identifying personality traits that you wish to have, practicing them diligently, and coming out the other side as a person who recognizably possesses those traits.
Suppose I wanted to be a kinder, friendlier person. Suppose I undertook to express kindness and friendliness to other people - to everyone - in a way I had never done before. Suppose I succeeded in changing my own personal definition of myself. In other words, suppose those kinder, friendlier traits I aspired to have became second nature to me through training, and vigilance, and reassessment, and doubling down, and so forth. Suppose I, a person not usually viewed as kind and friendly, did everything in my power to become a kind, friendly person.
How long do you think it would take for other people to notice? Would it matter whether they did notice? Are profound changes in personal behavior still significant if they go unnoticed by others? To what extent is external validation a requirement of personal change?
These are inward considerations, by which I mean, these are questions that we as individuals must wrestle with as we endeavor to grow as human beings. (And growth is inevitable. Even if you don't want to grow, you will.) Still, there is only so much a person can do. Beyond that, the rest is up to others.
I think this is why it is so important to, if not forgive, give people a chance to change.
Not everyone, of course - there are some people out there who may have hurt you so deeply that you no longer have any reason to give them additional chances.
But for everyone else - those who have not hurt you especially badly - I think it's important to at least be receptive to the possibility that they may have grown, they may have changed, they may have turned over a new leaf. They may have done all the work on their side to improve their character, but what will it matter if you hold a grudge? By the simple act of expressing skepticism at their ability to change you have provided them a marginal incentive to go back to being the person that they were.
Maybe that's why forgiveness has always been considered a virtue. It's important to give people a way out of being villains; if we don't, then they may very well resign themselves to their villainy. I think that's what happens to some people who are criminally mentally ill, but if so, then it certainly must also happen to the rest of us, only to a lesser degree.
You must know someone who wants to turn over a new leaf. Do them a favor and give them a chance. You never know.