As people progress and reflect through life, the people around them often help in shaping experiences and perspective. Because of that, choosing the right people to be around is an essential asset to quality of life. In this sense, people are much like microbes. There are all different types.
Some will develop a mutualistic relationship with you. They will learn as much from you as you from them. You will grow together; and in that, both learn and benefit from each other. These are the life long friends. The friends that will support you and go through life with you. These people are essential to happiness and you don't want to lose. There is a mutual establishment of love and respect, as well as, a strong foundation for the future. This is the ideal relationship to develop and look for in friends.
A mutualistic relationship does not mean that you will always be happy and never disagree; but it is valuable to be able to realize the character and meaning behind action and disagreement. Not everyone will agree with you one hundred percent; and that is significant in growing and developing persona. Mutualistic people will challenge you and allow you to challenge them, that is how internal and mental growth is established.
In contrast, there will also be people that are parasitic. They will appear to be good for you and once they get closer they will benefit from you while disregarding how it can affect you. Parasites come in all shapes and sized; and once in a while are the people hardest to become rid of. They will bring you down and hinder potential, taking from your character and thriving from your fall.
Through experience and objectivity it will become apparent who belongs to each category and where they should be placed in your life. Placing people and realizing how people affect your life is important; and like all important things, can be difficult to put into practice.