Steinbeck presents Lennie as a childlike man. Lennie is simple-minded. Next to George, Lennie is a large man:
Lennie is 'his opposite, a huge man, shapeless of face, with large, pale eyes, with wide, sloping shoulders.'
He would never hurt anyone intentionally. Lennie doesn't know his own strength. He is strong and heavy handed. He is mentally challenged:
Lennie is mentally handicapped: he cannot quite remember what had happened in Weed; he speaks with a child's vocabulary; and he bursts into tears when George makes him give up the dead mouse that he has been secretly petting in his pocket.
Lennie would never have broken Curley's hand if Curley had not instigated the fight.
Likewise, Lennie only meant to quiet Curley's wife. He never meant to break her neck.
George loves Lennie unconditionally. Lennie loves George. The two of them are as close as family is. Lennie trusts George. He has lived by his every word.
With simple trust, Lennie has no idea what George is about to do. Lennie would never have believed that George was about to shoot him. George meant everything to Lennie.