My best friend and I like each other, and we decided to just be friends. Why do i feel this way?

My best friend is a boy, and we're still fairly young. We've been friends for two years, really close this last year. I liked him at the end of last year, for three months, before I found out he liked a girl named Chloe, one of my friends. Chloe knew that I liked him, but she has known him longer, so I didn't want to cause a fight. They went out for six months, Luke being nothing but sweet, compassionate, and adorable to her. I went through my few boyfriends in the period, and Luke stayed close with me. A bit into our friendship, Chloe changed. She started spreading rumors about me, getting jealous that luke would rather hang out with me then her(She asked if he wanted to see a movie and he said he was hanging with me), and just becoming plain mean. Then Luke broke up with her. It was sad, and she's still not over it(only been 2 months). I learned, fairly late april, that Luke liked me, and that one of the reasons he broke up with Chloe was because he did, and he hated how she was treating him. We talked about it awkwardly, and decided we wanted to stay best friends. Just friends. We had something good going on, being friends, and I know that since we're young and feeling of "love" for us is just our hormones flaring up, meaning nothing really lasts long in our age. But why, even with my reasoning, do I feel like screaming and breaking our plan or idea every time I see him smile? I want nothing more then to be his girlfriend, even though I know that it would jeopardize our very friendship and life. I would never want that to happen, but... I'm just so lost. So of course, I turn to Yahoo Answers. *Sigh* Anyway, what should I do?