I want to know if there's a way to prevent that sagging skin? why does it happen for me just under the eyes?

Dermatochalasis: Is an age-related condition in which the eyelid skin and underlying muscles become stretched out and sag. This tends to run in families. It is not damaging to the eyes, but there is no medical treatment for this. Some people have surgical removal of this excess tissue. The procedure is called a blepharoplasty.

True prevention: Is not really possible because we cannot stop the aging process, but it can be slowed by not smoking and min. Sun exposure. Once a person has this "loose skin" we look at treatment options. Depending in the location and severity of the affected area, options might include chemical/laser resurfacing for minor tightening of the face vs blepharoplasty/surgery. See an exp. Plastic surgeon for exam/rec.