general relativity - The bigger the mass, the more time slows down. Why is this?

If I were to stand by a pyramid, which weighs about 20 million tons, I would slow down by a trillion million million million of second. Don't know if that's exactly right, but you get the point. Also, if you went into a black hole, time would stop at the event horizon (as measured by a clock of a distant outside observer). Why is this?