Someone who is a hopeless romantic is someone who has unrealistic expectations about love and life.
Things are not always easy, and do not always have a happy ending. You have to work hard for things in life and with love. Sometimes your knight in shining armor turns out to be someone who is just using you. And you start to realize that love at first sight does not exist.
Hopeless romantics tend to have unreasonable expectations, which cause them to destroy relationships. Hope that answered it for you :)