I feel like most guy-girl friendships do revolve around some sort of physical attraction, either between both or just one of the friends.
I know it sucks, but even if you are desperately in love with the girl - if she only views you as a friend, it's still better to just remain friends then try to push the "friendship" boundaries into a girlfriend-boyfriend relationship, thus ruining the friendship in the process.
If you truly care for the girl, then you want her to be happy - and she's happiest with you as "just a friend". Just put your physical desires aside and enjoy her friendship in return.