The first thing to note is that, throughout Of Mice and Men, Curley's wife's name is never given. She is treated by everyone, most notably Curley, as Curley's possession. She is described as a flirt, a mere sexual object. And although she does flirt, this is partially her attempt to seek out companionship. She is similar to Crooks in this way. They both are relatively isolated; he, the only African-American on the ranch and she, the only woman.
Curley's wife is stuck in a bad situation. This is clearly not the life she had envisioned for herself. The same could be said for many of the characters. Life as an itinerant ranch hand is hardly desirable. The novel is set during the Great Depression, so her decision to marry Curley might have been based on the fact that his father owned a business and therefore, Curley came with the near certainty of financial independence.
In Chapter 4, Curley's wife tries to talk with Lennie and Crooks. Curley is at the whorehouse with the others. Here is another reason to sympathize with Curley's wife. She threatens Crooks when he tells her to leave. Crooks knows that she, being the wife of the owner's son, can get him fired so he backs down.
In Chapter 5, when Curley's wife approaches Lennie, she divulges some of her regrets.
Coulda been in the movies, an' had nice clothes--all them nice clothes like they wear. An' I coulda sat in them big hotels, an' had pitchers took of me.
In the end, whether she was seducing Lennie or just making trouble, her death is still tragic. She was an innocent victim of Lennie's innocent but destructive psychological makeup.