How does Of Mice and Men by John Steinbeck appeal to such a wide audience?

Steinbeck's Of Mice and Menis a universally celebrated story because of its deep ties to the human experience. The relationship between George and Lennie is brotherhood, is human connection, at its most simple and powerful. Remember at the beginning of the story when George and Lennie are talking by the river? They're sitting on the bank, discussing what their perfect future life would look like. Lennie bugs George to tell him about the farm...

Steinbeck's Of Mice and Men is a universally celebrated story because of its deep ties to the human experience. The relationship between George and Lennie is brotherhood, is human connection, at its most simple and powerful. Remember at the beginning of the story when George and Lennie are talking by the river? They're sitting on the bank, discussing what their perfect future life would look like. Lennie bugs George to tell him about the farm they might one day own and operate together. George agrees, and describes how Lennie will be able to care for the animals. 

That relationship is why the story is so popular. George's care for Lennie is total, despite that fact that, without Lennie, George's life could be "so easy."  He's willing to do whatever it takes to take care of his friend--culminating, of course, in that painful climax. 

It is in George and Lennie's friendship that we readers see ourselves, or what we long for. That level of human connection is rare, and we all want it. Maybe reading the story makes you think of your spouse or significant other and how you'd do anything for them. Maybe it makes you think of your best friend or your sibling. Maybe it makes you realize that you don't have that kind of connection with anyone, and sparks a longing in you. 

This is what good literature does, it shows us what we want most deeply.