This is a fundamental question. It has been described mathematically,
but not in an intuitive mechanical sense. I have been working on a
neutrino aether theory which may help explain the mechanical reasons
why this happens. In my theory, all of "empty" space is actually
filled with an ordered checkboard array of matched protons and
electrons. I am calling these matched protons/electrons "neutrinos"
for lack of any other suitable terminology to describe a dipole
binding of proton/electron, but may not actually be the same as what
is commonly known or accepted as a neutrino or neutron. In any case,
if space were filled with these neutrally charged particles, they
would all exhibit an attraction to a strong electrostatic force. My
theory postulates that gravity is simply a positive electric charge on
the Earth such that the Earth acts as a large positively charged
sphere is space. I have many posts discussing the details of how this
can work and how the Earth and Moon don't go crashing into each other
(search frankli...@yahoo.com). If this is true, then the density of
the aether will be higher in a higher gravitational fields due to
compression forces. Just like water pressure in the deep sea is higher
than on the surface. With this higher density means that any
mechanical process from point A to point B will have to traverse more
aether particles than it would in a lower density area. The increased
number of particle interactions effectively increases the amount of
time (in an absolute sense) that it takes for something (like the
ticking of an atomic clock) to happen. Thus, clocks in a high gravity
field tick slower than one in a lower gravity field. This is a simple
and intuitive mechanical explanation that shows that rate that time
passes is a function of the local environment.