If you're talking about in-game, then it could be any number of reasons. Maybe she's mentally ill, maybe she wants attention, maybe she's actually a robot... could be anything, really, as we never get a definitive answer. However, if you're looking for a reason as to why Game Freak did it, then it's most likely what sumwun said, to make her stand out.
EDIT: I went back to her Bulbapedia page to double-check, and it appears once you clear the Delta Episode and make amends with Maxie so that he'll use Team Magma for good, Courtney breaks down crying and apologizes, and after that, her dialogue is perfectly normal, so perhaps she was so devoted to her cause and her leader that it affected her thinking and speech, but like I said above, we never really get a definitive answer, so its impossible to say.
