Edited sentence: Although she resented not getting a Hogwarts letter, she still retained her love for Lily.It is conveyed earlier in the series that Petunia never hated Lily -through the letter Lily sent Sirius.It is implied that she loathed magic because Severus (a magical child) got Lily's attention as days progressed from the 'swing' scene but if a keen reader reads between the lines, he can really decipher what the main reason was.Reason why she hated magic.That's the true reason why she never treated Harry well, and pretended magic never existed.The main reason Lily got killed was magic - then they got landed with Harry (also seen in the Philosopher's Stone movie although just unintentionally placed).They communicated and even sent a vase to Lily after Harry's birth