The concept of personal development is nothing new. We used to call it maturing or growing as a person. It begins at birth, when a baby watches and listens to everything around them. They are like little sponges soaking up knowledge.
It is a natural struggling to grow and develop. Consider the ease with which they learn their first language. And if they are exposed to more than one language they will easily learn them both.
But, as we age this natural development becomes less natural and more effort is involved. We have created schools where we are lead in our development according to the norms of our society. Our parents direct our development by laying down guidelines of behavior according to their standards and morals.
Still the time comes when we must decide what direction our life will take. This is when our development becomes more truly 'personal'. If we do not make a conscious effort at this point to grow and mature, we will become stagnant. Our happiness will be in jeopardy. People will view us as 'unsuccessful' in life.
Really the development of our self as a person would include making our self more useful to the people around us. We have seen many famous people who seek to do this through charitable works. They will often say how those activities have enriched their own lives!
It is a bible principle often repeated that there is more happiness to be found in giving and serving than in be served or in receiving. When we learn this fundamental truth we will be adding to our own personal development.
If we can interact with cultures that are different from our own this too will add to our personal development. It will cause our brains to stretch and learn, just as when we were a babe. Because we will begin to think outside of our 'box' so to speak. What we may think of as normal may be quite unusual to a person from another culture and vice versa.
I have often thought that most Americans are limited by their lack of interaction with other nationalities. There is mass migration of peoples all over the earth today. Thus it is become quite easy to find peoples of other lands with whom to interact. Your personal development will be enhanced by such experiences. The Europeans have been benefiting from such cultural exchanges for centuries with out losing their uniqueness.
I have seen that the best form of personal development comes from Bible study. The application of the many wonderful principles
contained in it will help you to become a better person. In Jesus sermon on the mount alone he instructed us on the best way to deal with arguments. How to make peace with another person, how to make your marriage strong. How to view material things in a way that leads to happiness.
Yes the best aid in personal development is spiritual knowledge.