Experience is a great teacher. When we are young we tend to have a lot of wonderful ideas about the world. As we grow older and more experienced, suffering let-downs and disappointments and heartbreaks, we lost some of our idealism and gain a little more realism.
Too much realism, however, can be dangerous. Excessive realism can make you cynically pessimistic. On the other hand, excessive idealism isn’t good either. Too much of it can make you naively optimistic. As with most things, balance between the two is best. You see clearer when you use both your eyes. Having a good measure of both idealism and realism is important if one is to have a clear view of the world and understand how it works.
A good way to develop both realism and idealism is to learn from the experiences of others. Read political history and accounts of war to develop your sense of realism. See how awful people can be. Read biographies and autobiographies of great people to develop your sense of idealism. See how inspiring people can be.
You see clearer when you use both your eyes.