I am in love with my best friend, but she already likes someone else. What should I do?

I have fallen for my best friend but she likes someone else. Am I right to pursue her or should I stay quiet?

I love my best friend. I think we have a very good chemistry. But as in most best friend stories, she likes someone else. My best friend absolutely loves me but is not hinting for a relationship. Not to brag but I honestly think I deserve her more than anybody else. The boy she likes is a very good friend of ours but I have never seen him showing a real interest in her. It hurts when you give your best to someone still end up on the losing side. Should I still have hope and go for her or should I just step down?