Sensory fatigue + selective memory.

Olfactory fatigue is when you get "used to" a smell and don't notice it anymore until you leave the room for awhile and then come back.

Our brains are continually making way for new and interesting sensory input by ignoring or setting aside the present. If one knows the punchline it's old news. But it can work on a new audience or as an "oldie" if it's been awhile.

Including hormones, mobile phones are one of the most powerful drug delivery systems ever invented. Attention span is now measured in seconds.