After she graduated she needed something to do to keep her mind off of waiting for NEWT scores, and she figured it couldn’t hurt her relationship to learn how to cook properly.
Within a couple months she had grown to really enjoy it — especially the first time she saw the look on Ron’s face when Molly let them all taste something before she told them Hermione had made it.
She and Molly had become much closer by the time the summer was over.