Skin has an ability to constantly shed old and dead cells and produce new cells, thereby repairing itself from wounds, cuts or incisions; when such a "repairing" process takes, it may also result in scarring. Thus, scarring is basically caused by an exaggerated healing response of the skin to an injury. Some scars may fade with time, while others bear the testimony to the injury for decades.
There are various reasons for scarring. Some of them include repeated motion, injury, surgery and certain medical conditions.
Injury-Induced Scars
When there is a deep wound on the skin, connective tissues and skin cells will work together to repair the damage. Fibrobalsts or the connective tissues form a framework in the injured spot on which the skin cells have to move and fill the wounded area. If the rate of replication of the fibrobalsts and the replacement by skin cells do not match, a dense network of connective tissues gets formed as fibrobalsts replicate quickly. This dense network which is not replaced by skin cells causes scarring. Certain scars fade with time because the skin cells slowly move to occupy this network.
Incisions of Surgery
The incisions and cuts caused by a scar start the natural healing process by the formation of fibroblast, adhesions and collagen so that the damaged skin cells can be replaced. When the fibroblast replicate to form framework even after the wound is healed, it gets protruded out of the skin, resulting in keloid. It is common in certain ethnic people or dark skinned people who had deep incisions in their surgical procedure.
Repeated Motion
When a specific part of the body is moved repeatedly, muscles of that area become tight. As a result of this, oxygen supply to the connective tissues and muscles gets restricted. Lack of oxygen can lead to the formation of scar tissue.
Burns
Within the first week of a burn, inflammation of the wound starts and it also begins to heal. Injured dermal elements get replaced by new epithelium within 2 weeks. However, an exaggerated inflammatory phase of the open or infected burn wound will result in keloid formation because it increases number of fibroblasts.
Hypertrophic scars are also common in deep burn cases, even after 50% of the wounds have healed. Such scars are reddish, raised, itchy, rigid and painful. They can also retard the movement due to pain and skin breakdown. This type of scar begins to reduce when collagen lysis exceeds the collagen deposits.
Acne Scars
When inflamed lesions like papule or pustule of acne condition get filled with excess oil, dead cells and bacteria, they break the follicle wall. If the lesion is deep into the follicle wall, then it destroys the healthy tissues and starts the wound-repairing process. This repairing process leads to scarring.
Scarring is thus caused by the natural repairing process of your skin. Although most of the scars fade with time, some can persist for a long time and may require cosmetic surgery or other procedures, like radiotherapy, to get them removed permanently.
