Initially Lennie didn't want to hurt Curly, besides the fact that he was beating him up. The only reason Lennie decided to fight back was because George told him to. It's not that Lennie is not capable of fighting back, but that he is a kindhearted soul, who cares for others and dreads the fact that he would ever hurt somebody. This is why after the fight stopped Lennie cried and kept saying that he was sorry. Steinbeck uses the words in the quote I choose to show the reader the true character that Lennie is. Steinbeck shows that Lennie cares for others by once again using this fight between Lennie and Curly. For example Lennie took Georges and his jobs into consideration when fighting and waited for the approval/order of George before retaliating
