The characters in OF MICE AND MEN suffer from the same desires and frustrations that the average person in society encounters. They are seeking freedom, independence, shelter and security. When people are missing things that they crave they sometimes punish those who are closest to them. George is the caretaker of Lennie because Lennie has no family. George himself has no wife or children. Perhaps, he feels burdened by his self-appointed job. George says that he would do cruel things to Lennie until he realized that it was wrong. Lennie almost drowned due to his blind trust in George. In addition, the story is set during the 1930s Depression Era. Like the characters in the book, many people at that time just wanted to have their basic needs taken care of such as food, shelter, etc. Due to joblessness and homelessness it is not surprising that the characters in the novel respond to hardship like real people.
One could ask this question all day and still not get a final answer. People tend to be cruel for many different reasons. In George’s case it could have been because he was young and Lenny embarrassed him or he could have been just plain annoyed with him. George may have felt frustration at having to be Lenny's care taker which led him to mistreat Lenny.
Some people mistreat others because they themselves had been mistreated, which is the reason that it is very hard to end cycles of abuse. There are others who feel an increase in their own self-esteem by hurting a person they consider to be weaker. Sometimes people are just mean and enjoy hurting others. Another reason for cruelty can be fear. If a person perceives that another person might take something from him he may be cruel. Then again, there are those people that love a good laugh at someone else’s expense.
Some people never learn to stop being cruel to others. However, in George's case I think that he matured and as he matured he became more accepting of the things that Lenny could not do. For some people having the reverse experience can be an eye opener. No one enjoys when he is the one being picked on.