It can certainly be argued that Lennie Small's motivations are nothing more than primal. Of his character, John Steinbeck himself wrote,
Lennie was not to represent insanity at all but the inarticulate and powerful yearning of all men.
This "power yearning" is for safety, comfort, love/fraternity, and the freedom to dream. Even an examination of Lennie Small's name reveals his nature as childlike. First of all, instead of the affix -y added to his name, as in most boys names such as Bobby, Joey, Jimmy, Charley, etc.--names which are often dropped and the more adult forms of Bob, Jim, and Joe assumed--Lennie has an affix normally attached to female names. In this way, Lennie's virile masculinity of strength and size is mitigated, thus allowing his childishness and yearnings to transcend gender. Of course, his last name of Small seems terribly ironic given Lennie's animalistic strength and size, but this nomenclature overrides size to connote that Lennie is, indeed "small" and basic with his childish mind and instincts and yearnings. For example, in the opening scene of Of Mice and Men, Lennie plods behind George like a child, "dragging his feet a little." And, in his childlike way, he plays surreptitiously with his forbidden toy: a mouse which he hides in his pocket.
When George scolds Lennie for having the mouse, Lennie, like a child, explains that he merely wants to pet it, just as he simply wants to pet the puppy and even Curley's wife's hair. Clearly, he has no motivations beyond the sensory level with physical objects. Further, in other instances, Lennie's motivations continue to be based upon the physical: comfort and safety. In Chapter 5 when, for instance, Crooks taunts Lennie with the idea that George may not return from town, Lennie's reaction is based upon the fear that his caretaker may not return, he retorts, "George wouldn't do nothing like that. I been with George a long time. He'll come back tonight---"
In particular, the dream of owning a farm on which he can pet his own rabbits appeals to Lennie's childlike desire for fantasy brought to life and his desire for security. Truly, Lennie's motivations throughout Steinbeck's novella are nothing short of "the inarticulate and powerful yearning of all men," elemental ones that lie within the heart of all men from their childhood: safety, love, comfort, physical pleasure, and a little bit of heaven in dreams.
Lennie is depicted in a very childlike manner throughout Of Mice and Men. Accordingly, he acts in a manner that is consistent with a child in that his motivations directly lead to his actions. He does not act in a pure sense of duplicity, reflective of the purity that is intrinsic to someone who is like a kid at heart.
The pattern of motivations directly leading to his actions can be seen throughout the novel. The first time the reader is introduced to Lennie is a moment in which his motivations directly lead to his actions. The desert wind that creates the heat which surrounds George and Lennie opens the novella. Steinbeck does not mince words in describe the natural conditions that exert influence on creatures, big and small: "Evening of a hot day started the little wind to moving among the leaves. The shade climbed up the hills toward the top." The timing allows those who are thirsty to find a source of water, something that might not be able to be done as easily in the searing heat of the day. The motivation to find a water source and drink from it is what causes Lennie to plunge his head inside the first "green pool" of water that George and Lennie find: "Lennie dipped his whole head under, hat and all, and then he sat up on the bank and his hat dripped down on his blue coat and ran down his back. “That’s good,” he said. “You drink some, George. You take a good big drink.” He smiled happily." Lennie's motivations of needing to quench his thirst is what drives him to drink from the small pond with so much vigor. George expresses his hesitation in drinking from the same source when he says, "Looks kinda scummy." Yet, to Lennie, this is moot. He is thirsty. He sees a water source. He dunks his head in it to quench his thirst. In this instance, motivations directly impact action.
Lennie's motivations are not merely physical. There are moments in which they are driven out of an emotional need to honor George or show deference to him. In the exchange between Lennie and Crooks, the issue of companionship is raised. Crooks lives a life alone and he possesses a sense of envy towards the friendship that Lennie shares with George. When Crooks wishes to make his point, he talks to Lennie about how his (Lennie's) world would fundamentally change if George left and went out on his own: "I said s’pose George went into town tonight and you never heard of him no more.” Crooks pressed forward some kind of private victory. “Just s’pose that,” he repeated." Crooks advances this in discussing with Lennie the implications such an action, suggesting that George might simply fall victim to getting hurt. However, Lennie interprets this vision as a threat of harm against George: "Suddenly Lennie’s eyes centered and grew quiet, and mad. He stood up and walked dangerously toward Crooks. “Who hurt George?” he demanded." At this moment, Lennie threatens Crooks through body language and voice inflection because of his motivation to protect George. The motivation of defending George initiates his actions of becoming aggressive, something that Crooks immediately realizes in backing down from his initial stance. Lennie's motivations to protect George inspire his actions in wishing to do harm to anyone or anything that would threaten him.