Petunia Dursley hated Ginny Weasley.
There were many reasons why, but it all boiled down to her sister.
She was headstrong, but with the same gentle heart Lily had.
She was a witch, with beautiful, vibrant red hair, although it was a shade lighter than lily's.
It still reminded her of her long dead sister.
When she sees her nephew kissing her, its like taking a step back in time.
Almost as if, James and Lily Potter are standing in front of her again.
However, what she hates the most about Ginny, is that she is stealing the only thing she has left of her sister.
Harry
When Harry was dropped on her doorstep, she vowed she would not let that other world take him like it took her sister.
Yes she and Vernon had treated him cruelly, but it was to force the magic out of him.
They'd only done what they thought was right.
They're efforts, however, proved to be in vain the other world had taken him.
While she pushed him farther and farther away.
Deep down, she didn't want him to leave, because he was all she had left of Lily.
Harry didn't just share Lily's green eyes, he shared her kindness, her compassion, her loyalty, and her bravery.
So how dare that little imposter, think herself good enough for her nephew?
Good enough for Lily's son?
She didn't deserve him
That's the main reason she hates Ginny Weasley so much, not for her red hair, manner, or background.
But because she doesn't deserve Harry, and is taking him away from her anyway.