Why do guys never get contented with their girlfriends?

cause as soon as we do, we give it our all, we care, we love, we give attention. then all of sudden, she doesn't want you anymore. Girls want what they can't have. it's why i always fail with a woman (cause i'm content) and why bad boys always win. women are emotional and they love that emotion of "wondering" if we care for her or not. women love the chase.