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.