The reason is that r expresses the extent to which paired scores are equally deviant from their means. Thus if you are 1 SD above the mean in height and 1 above in weight, your height and weight agree perfectly in the dimension free metric of deviancy units. Here are some things to know about r . Dimension free number, just like z 2. Not a measure of degree of relationship in general.
Regression analysis is a procedure for making the best possible prediction.
The procedure involves calculating the best fitting straight-line for a set of points. Regression to the mean. Correlation does not imply causation. Correlation is necessary but not sufficient for demonstrating causality. Annual oil production is related to the size of the world's population 2.