I'm not exactly what you mean by this question, but I'm going to assume that you're asking about the universality of the book or its universal themes. While this novel has several themes that are seen throughout (see the link below) there are some that stand out more than others.
Loneliness is a very prevalent theme for several characters throughout the novel and the way that each copes with it is different (see the 2nd link to the DB). This is universal because everyone can identify with feelings of alienation and isolation at some point in their lives.
Loyalty and friendship are other very prominent themes in the book. Lennie and George are loyal to each other even in the face of much adversity. They have a close friendship which is what feeds their loyalty. For George, loyalty is even tougher because he has to make several sacrifices, namely gainful employment, on Lennie's behalf. His biggest sacrifice was at the end of the novel when George kills Lennie to save him from the torture the men hunting him might have inflicted. Loyalty and friendship have a definite universality.
There are several others so visit the link below because all of them can relate to the world today. This probably lends to the fact that this is such a widely read novel.
Hey Clane can you please tell me about the link that you talked about?
Where is the link?
The American Dream
Many migrant workers like Lennie and George had a dream to make someplace their own place in a harsh and unforgiving world. Some, like Lennie, whole-heartedly believed in this dream and were optimistic about their chances in a harsh and cruel world. Others, like George, believed only half-heartedly in these dreams and sometimes gave up on them. Some of these migrant workers worked all of their lives simply doing jobs for the sake of working or to save up for their dreams, like Candy. In every case the “American Dream” of a perfect life with no worries and happiness all around, drove this entire movement and still pushes people. However, George uses this dream of a perfect world to instill hope into Lennie. George didn’t do this to manipulate Lennie but to shelter him. He knew that because Lennie was so naïve, he couldn’t handle the truth and harshness of the real world. George even tells Candy that he knew that they would never get their perfect little farm. Candy worked his entire life as a ranch hand never quitting and working hard. Steinbeck gives some hints that Candy, at one time, believed that he would live the American Dream. Stating this makes one wonder what the American Dream truly is then. The American Dream is motivation, a hope for a better, brighter, and bountiful future. Many accomplish this goal for their children and heir’s so that their children can have better lives that they did. George and Lennie worked hard for this dream, like a dog chasing its tail. However they did achieve one thing in this long and hard journey, true friendship.
the theme is about loneliness, friendship, and broken dreams
Dreams or wrecked dreams is a strong theme in this book. In particular The American Dream gone sour. Steinbeck was trying to show that many people clung on to an unattainable American Dream. The book's title is from a Robert Burns poem called "To a mouse." The line is:
the best laid plans of mice and men gang aft agley
"gang aft agley" is Scots for - often go wrong
In the poem Burns apologises to the mouse for disturbing its nest. He thinks of all the mice killed by well-intentioned farmers.
So George and Lennie's plans (or dreams) go wrong.
They want to own a place and "live of the fatta the lan." Steinbeck is showing that this is an impossible dream, although still a dream shared by millions. In a sense his universal theme is that many of us are living in some kind of fantasy world rather than facing the harsh reality of our lives.