In a fashion similar to an old over used elastic band collagen fibers under the skin surface break when overstretched. The result is atrophy of the deeper dermal layers of the skin & a grossly visible stretch mark. The stretching can be caused by large underlying muscles or accumulations of fat. This is very commonly seen on the abdominal skin of women after childbirth. The only way to get rid of the stretch marks is remove the involved skin as in an abdominoplasty/tummy tuck or thigh-buttock lift. In some people a flashlamp laser treatment will the make the stretch marks less noticeable but will not completely erase them.
For the shoulder & armpit area it would not be a good idea to excise the skin unless one has lost a lot of weight & is left with a lot of hanging skin. Thus, laser would be the way to go in this area. It doesn’t work in everyone but the cost is minimal, there is virtually no down time, the complications are almost non-existent in light skinned individuals & usually no anesthesia is required.