Consider a highway analogy. In a parallel circuit, each additional resistor is like adding an extra lane to the highway. In the usual circumstances, traffic (or electrical current) would be slowed down due to congestion (resistance), but as you add more lanes (resistors), each vehicle (electron) gains access to its own personal lane.
But here's where the analogy takes a twist. Instead of each lane simply providing more room for traffic, it also somehow magically reduces the number of cars (electrons) in each lane. So, with each added resistor (lane), the amount of electrical traffic (current) in each lane (resistor) decreases. This diminished traffic consequently reduces the speed limit (resistance) on each lane (resistor) because fewer cars (electrons) cause less wear and tear on the road (resistor), which is what the speed limit (resistance) is designed to prevent.
And there you have it: with each added resistor in a parallel circuit, there is less electrical traffic in each resistor, causing less wear and tear, which lowers the speed limit (resistance). The result is a decrease in total resistance in the circuit.