Maggie simply wants to make it in life and to stay far away from the poverty that haunts her youth. We see her fight against Gooper and Mae's wiles, against the disapproval of Big Mama, and, ultimately, we see her win a fierce family battle by (successfully?) ensnaring Brick. We root for her, and we don't quite know why.
While Brick renders Maggie's baby-making plans impossible, Maggie stands in the way of Brick's train to Echo Spring land (a.k.a. land of the Click). In Brick, we see a sad man who just wants to drink in peace, away from his weirdo family. Maggie simply will not give Brick the peace he wants and will not stop talking about Skipper. She is in many ways the catalyst (get it catalyst..hehe) in Skipper's death and in the unveiling of the love that exists between Brick and Skipper. Because of Maggie, Brick has lost the one true thing in his life. We are rooting for Brick to find that Click, but more importantly, we want him to free himself of his guilt over Skipper's death. It's starting to depress us.