Question

Why does time slow down near a black hole and stop completely inside it?

Answer 1

As per Einstein's theory of relativity, the closer the clock is to the source of gravitation, the slower the time passes.  Clocks close to massive bodies slow down.  The time difference between two events will be very large. Time difference between two clock ticks will take extremely large amount of period.

it is confirmed by tests of general relativity.
Black holes are strongly densed matter. the gravitational attraction influences the clock ticks. Atomic clocks run a little faster on aeroplanes , away from surface of earth. Very close to back hole, where attraction and gravity is extremely high, clock ticks so slow that time can be thought of as stopped...