its a process called habitutation and is a body response to stimuli when were are repeatedly exposed to a stimulus. It is something that occurs with all the body’s sense and is not exclusive to smell. Most research i am aware of suggests its a survival response used by the body to accommodate the constant barrage of sensory stimuli. If you never were able to tune out some stimulus your body/mind would become overwhelmed very quickly. This is also similar to why we just straight up tune out some things completely. Things your body is aware of as being unimportant or non threatening are tuned out to allow your conscious mind to perceive only those things that are the most important. Its basically your bodys way of being able to remain aware of environmental stimuli after its processed and become aware of certain things. Partially a survival mechanism and is present in most living organisms
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