User blog comment:Yatanogarasu/Will Hancock go with Luffy?/@comment-2117107-20100609221228

Remember, she has to think of her island's interest as well... Joining the crew would make her just another pirate again and lose her title as Shichibukai, which would endanger her people, as they are protected because of her position.

Also, she won't lose interest since it says that previous empresses had died from the love sickness, so why would she lose interest and get cured?