Acne scars, as its name suggests, is caused by acne, a skin condition that occurs due to inflammation of the sebaceous glands. During puberty, the sebaceous gland is activated and secretes sebum, skin oil, which lubricates the skin to protect against friction.
The inflammation of the gland causes excess secretion of sebum and makes it a propitious ground for certain bacteria to grow. This clogs the skin pores with bacteria, sebum, dirt, and dead skin cells leading to inflammation and scarring.