OMG a psychological reason why people dislike Sakura, they got they heart broken and turn that pain to hatred, hatred lead to a character that represents the girl that hurt their feelings, feeling empathy, somehow, with Naruto, a character that just cannot have a requited love by her beloved Sakura....Sakura seems to have a disproportional amount of haters, especially on the base, and I didn't really pay much attention to it until I watched a certain AMV then suddenly all of my questions were answered. Specifically the section from 2:10 until the end:
[video=youtube;OYYfjFtYGsY]http://www.youtube.com/watch?v=OYYfjFtYGsY&t=2m11s[/video]
Nice guys finish last. Sakura haters always complain about how she is "mean" to Naruto and still likes Sasuke. Saying she's "weak" and whatnot really doesn't hold any water considering she's one of the strongest of her generation outside of major plot characters and saying she's an "asspull" really doesn't fly either considering you have hawk-boy/fire-sword Sasuke standing not 10-feet away.
TL;DR: Forever alone dudes hate Sakura because she is the personification of the girl that always chooses the bad boy over them.
Summing it up, loosers hate Sakura, by dealing with their need of revenge with their actual heart-breaker.
Excellent theory!