Why does this particular employee irritate me so much?

This is one of those people that are always pushing boundaries. Then when I lay down the law she will start crying on me. Its to a point where I start to feel bad, like wondering if she is going through menopause or something. But just where do I draw the line? Should I **** can her or keep her? She is really starting to annoy the other employees to where they are coming to me complaining. But I feel like she has other things going on in life where I am feeling bad for her. Your opinion please.