Lennie, for God' sakes don't drink so much. You gonna be sick like you was last night.
You got no right to come in my room. This here's my room. Nobody got any right in here but me.
George and Lennie stumble upon a river, after a long walk. Lennie was very thirsty so he dropped himself and started drinking without hesitation. George scolds him, because he can get sick again. This shows that Lennie, most likely, isn't very bright and/or worried about his health. This also proves that George is like his babysitter.
I done a bad thing. I done another bad thing.
Lennie walked into Crooks bunk, wanting to make a friend. Crooks was very upset at first... They begin to have a conversation and Crooks tells Lennie all about his isolation due to his race. This shows, Crooks just needed someone to converse with after his time of being secluded. Therefore, loneliness is a destructive force and communication is key.
Curley's wife walked up to Lennie, she refused to accept that George didn't want Lennie talking to her and continuously asked questions, putting herself onto him. She offered him to touch her hair, so he did. After a while, she asked Lennie to stop, but he started to panic and wouldn't let go... Not letting go, led into breaking her neck. This shows, Curley's wife should have listened, instead of trying to put herself onto him. Therefore, you can't always get your way and expect a good outcome.
After murdering Curley's wife, Lennie ran to the bush George told him to go to if something were to go wrong. While there, he was scolding himself, telling himself George would be better off without him. Lennie started hallucinating, and saw his dead Aunt Clara. She spoke in his voice, but her words were as if someone was a bystander to all the events of him and George. This shows, Lennie does have a brain, he isn't stupid. Therefore, don't underestimate anyone, no matter how well you believe you know them.
You never give a thought to George. He been doin' nice things for you alla time.