The human olfactory sense (sense of smell) is what is known as adaptive. Example, when you walk into a room with an overwhelming, obnoxious smell, it is very "loud" at the beginning. But if you have to stay in that room for some reason, after awhile the smell is only a little or not at all (to you), but someone just walking in would comment on the strong smell, and you and others that had been there long enough would say "Oh it's not bad" or "You get used to it" or "Gee, I don't smell anything". And, of course, the newcomer looks at you incredulously, hardly believing his ears as he is nearly gagging it is so bad. Why our sense of smell works that way, I don't know, I just know that it does. (physiology class info).