It is perfectly normal to make a great bond with the b/f best friend. And romantic feelings are usually right close to the top--in situations like this, and of course they are the listeners, and you can talk to them more easily--always is that way. It is always that way. Or unless you hate the best friend. It is either love all the way---or hate all the way. That is just how it is!!!! It is so normal to form this bond. Now if bestfriend had a wonderful girlfriend you all would balance each other out. He(b/f) could go to her, and you could go to him(best friend). Unspoken love, unspoken bond, unspoken rule!
Now, you have to ask yourself if it is just a comfortable situation because there is no committment, or if the feelings are stronger than for the b/f. And remember it may go completely the opposite, when the b/f is outta the picture. It depends on his committment to his friend. Sometimes they may even have the same feelings as you--but will not persue it due to the loyalty they have for their friend.
And it may just be that normal bond that we build with those close to us, and close to those who love, who we love. Maybe your affection for him is because of the way you feel about your b/f.????
I hope I have given you something to think about. Good luck in the decisions you may make.