Many of the characters in John Steinbeck's Of Mice and Men experience suffering in a variety of ways.
Two types of suffering experienced by multiple characters in the novel are occupational suffering and the unattainability of the American dream. Many of the characters spend their lives working hard on other people's farms. They spend so much of their lives toiling away and only ever make just enough money to survive. They are not able to have their own property or live comfortably. The novel ends with neither George nor Lennie reaching their shared goal of owning their own farm.
George suffers because he feels obligated to honor a promise he made to Lennie's grandmother to care for Lennie after her passing. Lennie is mentally a child and is therefore completely reliant on George to guide, teach, and protect him. This is often a burden for George. Lennie does not understand consequences and makes poor decisions which have direct, negative impacts on George. For example, when the two worked at a ranch in Weed, Lennie grabbed a girl's dress and was accused of rape. As a result of Lennie's actions, the pair lost their jobs and had to flee from the police, as well as an angry mob. Because he must care for Lennie, George is not free to travel and work where he pleases, or pursue a romantic relationship.
Lennie suffers due to his lack of intelligence. He is not able to care for himself and even though he is chronologically an adult, he has the mind of a child and fully relies on George to care for him. He has a very limited understanding of consequences and, as a result, often inadvertently brings punishment upon himself and George due to his poor judgment. He does not intend to cause trouble, but trouble seems to always find him. He does not want to hurt or disappoint George, but he often does. The most powerful example of this is Lennie's accidental killing of Curley's wife, which results in George and Lennie losing their jobs. George is ultimately forced to kill Lennie to spare him from what would surely be a brutal death at Curley's hands.
Curley's wife suffers because she is in a bad marriage and is lonely. She and her husband have a toxic, dysfunctional relationship. Curley is controlling and forbids his wife from talking to the men on the ranch. He does not allow her to have friends or socialize. She spends much of her time trying to get attention from the ranch workers to fill the void in her life.
Curley suffers because of his quick temper and controlling nature. He does not trust his wife and lives in constant fear of her cheating on him. He is constantly checking up on her to make sure she is not making friends or talking with men. He often threatens and starts fights with people he perceives as threats. He is very insecure and feels like he is not manly enough.
Candy is another character who experiences suffering. He is old and physically impaired. He lost one of his hands on the job and now fears that he will be fired because he is not physically able to work as much or as hard as he did when he was younger. One of his only joys in life is his dog, which, much like him, is also old and disabled. This last bit of happiness is taken from him when Carlton shoots the dog because it is no longer able to work as a sheepdog.