Q:
Why is time influenced by gravity?
A:
Einstein's theory of relativity showed that space and time are not independent. One consequence of this is that time can appear to pass more rapidly or slowly for two different observers depending on their relative velocities and acceleration. According to the theory of relativity, acceleration and gravity are equivalent, so gravity can affect the flow of time.
One way to look at this is to view gravity as due to a warp in space. Light loses energy as it climbs out of a warped region of space. This shows up as a lowering of frequency of the light waves, which means that, to an observer far from the warp, atoms appear to vibrate more slowly, clocks run more slowly, etc. in a strong gravitational field. To an observer near the warp, all things out in space appear to happen more rapidly.
For more about Einstein's theory of relativity see: http://chandra.harvard.edu/resources/faq/astrophysics/astrophysics-3.html.
Revised: September 15, 2008