Acne usually begins during puberty when androgens, sex hormones, begin to be produced and stimulate the sebaceous glands to produce more sebum than necessary. Acne can also occur later in life, because of, for example, a hormone imbalance. An excess of sebum, along with insufficient exfoliation of old skin cells makes the possibility of forming a stopper in the hair follicle more likely - a blackhead or pimple has then occurred. Fattier skin also often has an excess of bacteria that can cause inflammation in the blocked hair follicle, which can lead to a pimple (also called papule/pustule). These inflammations can become very deep, which can lead to cysts.
Inflamed acne in the skin also develops scars or pigmentations more easily after you squeeze the pimple. UV light can cause inflammation of the skin, causing more outbreaks. Use UV protection daily. Also avoid hard scrubs that spread the acne bacterium and accelerate the inflammation. Acids that exfoliate gently and down into the skin are a better option.