It stemmed from her rivalry with Ino. She liked him when they were around 8 years of age, because all the other girls did and Sakura herself was striving to fit in, she wanted to be like them. So, she liked him.
Also, in the manga for her confession in part one, she said that she came to love him. So, it started as a crush, and later evolved into love in her opinion.
To be honest, I don't see why a girl would love Sasuke when a) they've only spent less than a year being on a team with him, b) He shows no interest in anyone bar Naruto, and c) Naruto is there to support... but hey. Just my opinion. If Sakura wants to continue after Sasuke, she's more than welcome to do so. I just won't like her for that. I think she should have dropped it long ago.
I think we know enough about Sakura to understand her thoughts. Kishimoto wrote that he was bad with writing female characters and found it difficult. Sakura is emotional and kind at times. Her main fault is that her emotions get in the way when it counts. Also, that she let the idea of romance ruin her shinobi career slightly... if she had spent time training from the get-go, she'd be stronger than she is now. She should also have a more varied arsenal at this point as well, but she doesn't. She lacks the speed to land a hit on top tier opponents and be a genuine threat. She's just a mini-Tsunade. It's sad. She seems complex because she shows a wide range of emotion, however, this is due to Kishimoto not knowing which way to go with her and using her as a plot device to push the idea of Naruto's and Sasuke's rivalry. Her actual themes though "Women should be strong" and "Team 7 should be back together like the old days" are very simple and bland. They aren't bad and are actually nice... but, they lack complexity themselves.
And your sig.