This happens to both men and women all the time. Each person experiences this at least once in their lifetime. I side with the answers you've been given above. This usually happens when you have so many fantasies about that person; its like, in your mind you already pictured him to be/act in a certain way and unfortunately because it didnt turn out to be the way you thought it would, you feel disappointed.
But you need to open up your heart, use your mind's eye and see through him for who he is and not what you imagined him to be. Cos you never know; may be what you felt for him really still exists, but cos you had your mind focused and clouded by one thing, you can't seem to find it. Remember what about him made you even have him as a friend; what made you attracted to him; what about him made him different from other guys you had as friends, for you to have singled him out from the rest.
Another yedda user lovedoctor71 referred me sometime to this website. There could be some information in it that could help re-surface those feelings within.
http://romantic.adoringyou.com/But if you still think you can't/don't love him, feel the same way around/with him, you need to let him know about it, so that he shouldn't "build castles in the air" --- having hopes, dreams and making plans for both of you for the future. It will be a selfish thing to do, if you dont tell him in due time that you dont wonna be with him, so he could find the strength to move on. Remember, it takes time to mend and heal a broken heart. You need to be true to yourself and to him aswell. Wish you goodluck.