superstition. The Desire Sensor detects what rare item, or event you are farming
for, and prevents the game from giving it to you. Only when you forget about it, or no longer need it will the item drop, or event happen.
"Holy shit the Godsword Dropped! I remember running this dungeon for months, and I never got this damn thing. Of course now its worthless. Friggin Desire Sensor, why do you tease me so."
-MMO player's lament