Joseph Campbell and Bill Moyers make some interesting points in the fifth chapter of The Power of Myth. The first one is that all the myths about a hero's journey, including the three major religions of the Western world, say that there can be no reward without sacrifice. The main reward that readers look for, although most readers don't realize it, is that the main character loses a great deal, enough to send him through some sort of journey, and learns that what he lost was so much more than what he (or she) gained in the end. The person accused of murder finds a true friend they never knew they had in their partner or someone finds their true love after a hard journey. Often a person learns to forgive themselves or to love themselves or how to let go of the past and move on to a happy future after a hard mental journey. Many readers don't realize that the real journey the main character is on in any good book that has stood the trials of time is a mental one leading to the climax, whether it's standing up to the villain or kissing their beloved or feeling welcome in their family or friends again after a long time away. In Moby Dick Captain Ahab couldn't let go of his past obsession and it killed him. In Emma she realized that love in a marriage was better than a wealthy husband and she let her friend marry the poorer man Emma had been avoiding the whole book. Even movies show this reality of stories when you look at Star Wars and the first Avengers movie. They had to move on from their insecurities and past experiences in order to conquer the evil threatening what they knew. They talk some about the two types of heroes: the one that purposely sets out on a "quest" like most of the heros in mythology and the one more common in real life, the one who doesn't chose to be a hero but it happens. Their example of the second type was the soldier that got drafted. He/she didn't chose to go to war but their number came up and they had no choice. They still become a hero but they didn't chose to set out on a quest. In fact, many characters don't realize they are on a quest, like the main character in the movie The Game who thinks he is on a safe Hollywood treasure hunt while really having switch places with a mob hunt, or they drag their feet the entire time, wondering "why me, why me?" and complaining until they decide to do something about their predicament. All those people are heroes in the end that go through and internal journey while moving through the plot, they just started out differently.
Another thing Campbell comments on is that the villain in a story depends on the perspective of the reader. Some villains are evil because they want to be or because they were hurt and believe the hero deserves to be hurt. However, most real people who are seen as villains see themselves as heroes. Terrorists are furthering what is right and those who get hurt are standing in the way of "progress" or are hurting "the cause". From the point of view of most villains, they are morally correct and therefore not doing anything "wrong". It's how so many "evil" characters can continue doing hurting people for years. They're god is protecting them or they are creating a better future. Can you imagine watching the some of your favorite movies from the villain's point of view? In that story, the hero in the regular version is the antagonist in the villain's version. It is definitely something to keep in mind while you create your characters.
Campbell also talked about the story of Icarus and Daedalus and the wax wings. He compares Icarus's urge to fly too high to the modern idea of getting really excited about a new adventure or idea. How many times have you had an awesome idea you really wanted to tell everyone about before you sat down to start it? I often tell people about my great idea, only to sit down and not have it turn out at all like I'd hoped and told everyone about. The choice then is to decide whether it's worth continuing on to strive for days, months, years on the idea or give it up. Often the initial enthusiasm means that details can be overlooked and science imperfect. He also mentions that science is proving what myths have told us about. How many times have you heard about a scientific discovery and assumed it's merely urban legend?
Something else Campbell talks about which I have often realized. Our past has made each of us who we are. Often people can't accept their past, they hide parts of it or refuse to speak of things or even large parts of their lives because of shame or pain. However, our individual pasts, the friends we made and things we did, are at least as important to our development as the temperaments we are each born with. That is one reason why twins are never carbon copies of each other's personalities. One twin was closer to one friend than another, one twin had a teacher that the other twin didn't. A lot of stories deal with people finally dealing with their past shame or pain and once they do deal with it, once they accept what happened to them and that they are exactly what they should be, only then can they live full lives in the light instead of hiding partially in the shadows.