Without George's direction and instruction, Lennie is like a child left unattended. Lennie does not know how to behave in certain social situations. Lennie doesn't know his own strength. And when Lennie feels awkward, out of place, or that he's doing something wrong, he panics. These factors, coupled with Lennie's incredible strength, have led to trouble in his past. At the beginning of the story, George and Lennie have just left a job because Lennie panicked when he was left alone with a girl. In Chapter 3, George relates the story to Slim:
Dumb bastard that he is, he wants to touch ever'thing he likes. Just wants to feel it. So he reaches out to feel this red dress an' the girl lets out a squawk, and that gets Lennie all mixed up, and he holds on 'cause that's the only thing he can think to do.
The same situation occurs when Lennie is left alone with the puppies. He kills the puppy by being too rough with it. And when Curley's wife asks him to feel how soft her hair is, Lennie can not resist and he panics when she tells him to let go. He puts his hand over her mouth and nose because he doesn't want to get in trouble and inadvertently kills her.