She needs to clear her head because of the headaches if she doesn't. Because of the way doubts and strange visions of a different past cloud her mind if she doesn't focus. Because of the sharp downward spiral her thoughts will take if she doesn't keep them regimented this strictly.

In short, she needs to clear her head for many of the same reasons as Cortana.

As for a man... Mercury seemed to consider that a moment. She didn't, often, scream or yell around Kanda (aside from back soon after they'd first met, when he'd thought she was that other person with her name). But the fact that he came to mind, that meant something, didn't it? It meant that she had someone?

"Maybe I do," she said, but it was more to herself.