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Because we act like antennas and pick up stray voltages. Now, when we touch the plate (which has a high input resistance - NOT damping the signal voltage too much) the voltage from out bodies enters pin 2 bringing its level above and below the trigger point.
Perhaps try the low power version of the 555, the ICM7555. Then increase R3 to 10M or even 20M. This will make it much more sensitive and therefore help trigger!
