Why would he choose her over me?

I'm just having alot of trouble moving on. My guy friend and I knew eachother for 2 years. He always hinted that he liked me more than a friend and over the summer i started developing feelings for him too. The more i got to know him, the more things I discovered that we shared in common, and the more I liked him. Our future looked promising and I was convinced we would soon become a couple. Until this other girl came into the picture. All of a sudden one day he started talking to her, and over a weeks duration they became instant friends and he was always around her and ignored me. So I forced myself to move on since he seemed to be too enthralled with this other girl. Those few weeks that we held tension between eachother is when him and that other girl started a relationship. I did not expect that at all. He only knew her for a few weeks, and yet he knew me for 2 years! I dont see what he sees in her: she's so superficial. He deserves better. Me and him had much more of a history with eachother and we really related to eachother. We are back to being friends, but its not the same as it used to be. It frustrates me that he chose her over me! Was it because I wanted to take things slow and wasnt being foward enough? Or did he just lead me on without realizing it? I just don't get it.