Right from the start of the novella, Steinbeck likens Lennie to a large animal.
He walked heavily, dragging his feet a little, the way a bear drags his paws. His arms did not swing at his sides, but hung loosely.
The comparison is not meant to be derogatory, but actually descriptive of Lennie's primitive and unpolished nature, which exists within all of us as humans. Lennie, however, lives with the mismatched combination of a large body, disproportionate strength, and very low intellect. Hence, he lacks the typical behaviors and defense mechanisms that are used by other people to control themselves in certain situations.
Consequences are also foreign to Lennie. Like an animal, he just reacts on the spot without thinking much. For example, when he felt thirsty on his way to the ranch, rather than looking for a safe source of water, he went to the first puddle he found.
He flung himself down and drank from the surface of the green pool; drank with long gulps, snorting into the water like a horse.
In all, Lennie is no different than a wild creature that can only manage to survive by learning the basics of life. He is still a potential victim of his own instincts which, like those of a animal, command his actions more than common sense.