In "King Lear", why is Goneril angry at her father?

Goneril is angry because her father seems to like her younger sister Cordelia more than her or her sister, Regan. She says, "He always loved our sister most; and with what poor judgment he hath now cast her off appears too grossly" (I.i.290-92). Obviously, Lear has shown a preference for Cordelia all their lives and, understandably, Goneril resents this. At first, her anger can be understood as a reaction to poor parenting. However, by the end of the play, Goneril treats her father so cruelly that the audience is forced to sympathize with Lear. She even begins treating her husband poorly and calls him a "Milk-liver'd man!" (IV.ii.50). Thus, she allows what was probably a reasonable dislike over her treatment as a child to turn her into a miserable shrew.