The Kano Model customer satisfaction theory:

  Kano predicted that users' perceptions of satisfaction with a feature will shift from delight to expectation over time. This is either because they have got used to it or because competitors have started to include it in their offerings. In the case of the touchscreen, for instance, the newness that came with the iPhone is no longer a novelty; the touchscreen has become the norm. It's no big deal anymore, but taking it away would be a big deal!