Think of the X axis of the histogram as a teeter totter. Assume that the X-axis
itself and the Y axis have no weight.
You can think of each point's deviation from the mean as the influence the point
exerts on the tilt of the teeter totter. Positive values push down on the right side;
negative values push down on the left side. The farther a point is from the fulcrum,
the more influence it has.
Change the shape of the distribution and notice that if the fulcrum is placed at
the mean (portrayed as a small blue line under the X-axis), then the teeter totter
will be in balance.
Note that the mean deviation of the scores from the mean is always zero. That is
why the teeter totter is in balance when the fulcrum is at the mean. This makes the
mean the center of gravity for all the data points.