How could this happen?

Welcome to the lovely world of females. Sadly, I have seen this type of behavior even in older women. Seems they love to compete in a bad way with each other. That's why your best friends should be those of the male type (if you're lucky to fine one of those), or get yourself a gay male friend. I have met quite a few number of females who complain of the same thing you have described, so you're not alone. Matter of fact, I saw another post asking something similar.....By the way, from the guy's perspective, he is most likely thinking, 'wow, I got 2 females after me....' No loyal enough as you can see, since he said, he will have to think about it, as opposed to saying, 'i kind of like somebody else...' Anyway....best thing to do maybe is get rid of your so called best friend, as far as the guy is concerned, you can face him and ask him, 'so what have you decided to do...' Anyway, tough luck, but again, stay away from so called, 'best female friend.'