I believe struggle is the correct answer. Struggle.
In just about every piece of literature, some character is struggling with something. It could be outside forces, or it could be his or her own demons. It could be a struggle with his faith or her parents. Or a disease. Or maybe loneliness.
George Blanchine, the great ballet choreographer, once said that when you have characters on a stage, you have drama. That is because each one pushes and pulls the situation in a way that is different from the other person, and that creates a sense of drama -- how will this resolve? Who wins? Why? This is what draws a person in.
If we find the characters interesting, the struggle interesting, and the situation interesting, we are pulled into this drama because we want to find out how it will all end. Sometimes it doesn't, of course. And we are often feeling cheated if the ending is too pat. Crime and Punishment is a great example of a single character in a struggle with himself.
Poetry doesn't always involve a struggle. Sometimes it is just an observation, or a reflection of how one person views a situation. It doesn't have to be dramatic and if there is only one viewpoint presented, often it is not. So that would be the one caveat to my general comment that struggle is the overriding theme of literature.
https://www.quora.com/What-is-the-universal-theme-in-literature
