If your initial response to this statement is “why?” then the answer is simple - for the same reason that those who work in a sewerage plant no longer smell the poo. In other words, as you become accustomed to a particular scent it has a tendency to lose its effectiveness. It’s for this very reason that intermittent diffusing is a great way to maximize your products.
Throughout history various health conditions and ailments were treated with plants and herbs such as lavender; many of which are enjoying a resurgence in use today. In fact, one of the most reassuring things about essential oils (and the practice of aromatherapy) is their consistency over time.