Why? Let's look at the facts. She helped Sakura when she was little make friends and made her stop being such a cry-baby. How does she repay her? She turns their friendship into a rivalry over a boy, and not a great one at that, looks doesn't make a man, although Ino is also a victim of this for liking pretty boys, but she wasn't the one that started the rivalry, plus I think Ino has grown more than Sakura in general. Take the whole Sai scene for instance, how did Sakura react when Sai called Ino a pretty girl? Another fact is how Ino does more for her teammates in general, when Sakura was getting her ass handed to her by the sound 3, she got in on the action, Sakura has always been a coward and on the side-lines, plus she's manipulative and a liar. Shippuden for some reason made Sakura useful in the rescue Gaara arc, but that's it. Ino is also more out-going, Sakura has been stuck with Naruto, since they're a team, but Ino gets along with Naruto just fine, despite them barely knowing each other or even hung out. When Sai was first introduced, look at how Ino reacts when she sees Naruto, not something Sakura would do, that's for sure.