Steinbeck is emphasising Lennie's childlike qualities by this action, but there is a twofold reasoning. Lennie likes the comfort of soft things as a child would, but he is also a human with faults and failings which allow the reader to see Lennie as a fully rounded character. He is not a child. He makes choices which contribute to his downfall and the suffering of others. There is an innocence and vulnerability about Lennie; shown in his need for the fairytale which is the rabbit story George repeats. However, he has physical power beyond that of others. He is destructive, violent and dangerous, even if he does not intend to be.
Lennie is upset at having the mouse taken away, but his emotion is only really connected with pleasing George. He does not remember anything beyond what George tells him. This includes the past difficulties which he has got into.
Lennie is a vulnerable character but also a dangerous one. He is killed by George at the end to protect him from the lynching of Curley and his men, and the torture of the 'booby hatch'. However, he is also killed to end the trail of escalating violence which he has unwittingly begun.
The early dialogue between George and Lennie indicates the relationship between the two: George is Lennie's friend as well as caretaker since he has promised Lennie's dying aunt that he will look out for her nephew because she realizes that Lennie is mentally disabled. This mental disability makes Lennie childlike. He likes to pet furry animals, but his unthinking herculean strength leads him to pet too vigorously or react when they nip him and kill the animals in spite of his loving them.
Slowly, like a terrier who doesn't want to bring a ball to its master, Lennie approached, drew back, approached again....'I wasn't doin' nothing bad with it, George. Jus' stroking it.
This passage clearly indicates the dominance of George over Lennie, while at the same time it shows that Lennie does want to follow his own will. Here, too, is foreshadowing of further conflicts in the novella as it suggests that Lennie may disobey George, albeit unwittingly.
Another significant passage in the exposition of "Of Mice and Men" is in the repetition of their dream which becomes almost a mantra for them. Childishly, Lennie asks George to repeat their dream about the ranch for which they are saving. On this ranch Lennie hopes to have rabbits. After his scolding about the mouse, Lennie childishly resorts to talking about the rabbits as both consolation for and explanation of the death of the mice: "They [ain't] so little."