It certainly was not just a season 8 thing, it started in season 6. I actually liked the Amy/Willow rivalry. I liked how powerful he made Amy, she had that potential from the beginning, even before Willow started dabbling in magic. But Willow very quickly grew past her, and I think there was a lot of jealousy there. And resentment (that Willow hadn't turned Amy back human sooner, and that Willow didn't want to join in on Amy's games). It all came to a head in season 8, and I liked the storyline.