I am assuming that you are asking why did she go to basically butt in the barn where the men were staying. The answer to this is that she would always flirt with all the field hands because apparently Curly, her husband, is what we can infer to be a lesser type of man.
Knowing what we know about Curly, we know that he is arrogant, insecure, of a smaller size than Lennie (which drives him nuts), and loves to pick fights to feel better about himself. He also hangs from the fact that he is the farm owner's son, and so that makes him feel better than the others.
Curly's wife liked Lennie more than the other field hands because of his size and his simpleton ways. She tried to get him to flirt back with her but Lennie was not quite understanding her sleazy ways. In the end, as we know, Lennie accidentally broke her neck and killed her.