Steinbeck must have originally started with the idea of writing a story about two itinerant workers who had the dream of owning their own farm so that they wouldn't have to work so hard, so that they could have a home of their own, and so that they could be independent. But it must have occurred to him, as it has to many commentators, that a reader could easily form the impression that two such men were homosexuals. And Steinbeck did not want to write a novel about homosexuality. Such a novel would never get published in the 1930s anyway.
The conventional "dream" is for a man and a woman to own a farm together. This has been the standard since the start of the agricultural revolution in Mesopotamia and Egypt. The man and woman have children who eventually take over the land and provide for their parents, and life goes on like that for generation after generation. But Steinbeck couldn't write a story about a man and a woman traveling around together as itinerant workers, because women couldn't get such jobs, and they couldn't sleep in bunkhouses with a lot of men.
It apparently occurred to Steinbeck that he could make one of the characters mentally retarded and the other a man who looked after him. Steinbeck intended to convert his novel to a stage play immediately. He was using lots of dialogue in his novel to make it easy to adapt it to the stage. In a play the exposition has to be conveyed through dialogue. Making one of the men a retard would have an added advantage because the other man would have to explain everything to him and even repeat his explanations, which would be conveying information to the reader/viewer at the same time.
Thus Lennie and George were born. George has to take care of Lennie because of a promise he made to Aunt Clara, a rather nebulous figure from their past. Lennie is dumb but his creator makes him extremely strong to compensate by making it relatively easy for him to get farm jobs and to show that he would be an asset to George if and when they acquired their own farm.
Furthermore, it was Steinbeck's intention to have one buddy kill the other at the end. This would dramatize the thesis that the best laid plans of mice and men often go astray. Making Lennie a retard explains why he is always getting into trouble. Steinbeck needed Lennie to do something really bad at the end to explain why the mob wants to lynch him and why George shoots him to save him from being tortured.
Steinbeck establishes in the opening that Lennie is strongly attracted to soft little things but always kills them. He also establishes that Lennie is a liar.
Geprge said coldly, "You gonna give me that mouse or do have to sock you?"
"Give you what, George?"
'You know God damn well what. I want that mouse."
Lennie reluctantly reached into his pocket. His voice broke a little. "I don't know why I can't keep it. It ain't nobody's mouse. I didn't steal it. I found it lyin' right beisde the road."
Steinbeck's decision to make one of the bindlestiffs a retard led to his characterization of Lennie as dangerouosly irresponsible and to explaining the relationship between the two men. It also suggests how Lennie might get into trouble if his interest in soft little things leads him into the kind of trouble he had with the girl in Weed. Steinbeck also uses the opening chapter to establish that Lennie will return to this exact spot by the river and hide in the brush if he gets into trouble at the ranch where they are to start working tomorrow.