The journey of getting it is where most of the excitement comes from. The moment you open the box, it's just another iPhone with really not much changes except a different color. Your epiphany is really no different than going from 6 to 6s. Nothing life changing about it.
This doesn't just apply to iPhones. It can happen to other brands. Opening a new PS4 or Xbox One felt the same way. You shrug it off the slightly better graphics and go, what next? Boredom kicks in. Apple isn't the only one guilty of this. The hype and illusion fades. Back to reality.