Stretch marks are usually the result of the rapid stretching of the skin that is associated with rapid growth or rapid loss of weight. Stretch marks are also a common side effect of obesity as well as pregnancy. Read more on http://en.wikipedia.org/wiki/Stretch_marks.
I read somewhere that the marks are actually scars, just under the skin. Has something to do with the collagen tearing & not repairing properly, after weight gain then weight loss. ( Pregnancy & dieting.)