Of Mice and Men- Unfortunate misunderstanding

So Curley sees Lenny smile after he is getting into it with Carlson. Lenny is smiling because he is excited about owning a house having nothing to do with Curley. Unfortunately this provokes a fight between the two of them. It is really George's fault Lenny got involved. He told Lenny to do it. It is clear that Lenny doesn't think for himself.