At first, how does Crooks respond when Lennie mentions his dream to him? How and why does this response change?

At first, Crooks is deeply skeptical of George and Lennie's dream; he thinks the whole idea of him and George owning their own farm is just a fantasy. Over the years he's lost count of the number of men he's encountered with the exact same dream, and none of them have ever been successful. As someone who occupies the very lowest rung on society's ladder Crooks has years of bitter experience of what the world's really like. For him, it's a place of endless hard work, exploitation, and deep-seated racial prejudice. The very idea of being optimistic in such a climate is just incomprehensible to him.
Later on, Crooks briefly revises his initially hostile assessment in response to the goading of Curley's wife. One day she comes into the bunkhouse uninvited and starts mouthing off at Crooks, Lennie, and Candy. As an African American, Crooks is reluctant to answer back to a white woman, as he knows there could be serious consequences. But after some more goading from Curley's wife, he finally snaps and angrily lashes out, kicking away the nail keg on which he's been sitting. He then launches into a violent tirade at Curley's wife, in which he passionately defends the other men and their dream, claiming that they'll go get some land of their own even if she gets them fired. But after Curley's wife threatens him with a lynching, Crooks immediately goes back to his default position of subservience. He also snaps out of the fantasy he briefly harbored of joining with the other men and owning land of his own. In order to protect his injured pride and dignity, he claims that he wouldn't want to go to such a place anyway.