From what I gather, Sakura (indirectly) was part of the transformation into douchebagness for Shinji. From script when asked why they were friends, Shirou always said that Shinji wasn't that bad of a guy in middle school. But I think the constantly being reminded by good ol' gramps that you're a worthless mage so we got Sakura instead got to him. Plus I think there was plans to make Sakura conceive Shinji's child, think like if you have this girl you're suppose to marry when you're young. You watch that girl become quite the beauty...then fall in love with your friend.
All in all, I still hate him. (though I actually liked him in extra, him and Rider's exchanges were always funny.)