Acne / pimples occur mainly due to hormonal imbalances which causes an overproduction of oil and cells inside a skin follicle. They block the opening of the skin pore and cause the follicle under the skin to swell and become red.
Health Tips
What Makes Your Skin Prone to Acne? Top 5 Causes of Acne" by Dr. Jolly Shah Kapadia - https://www.practo.com/healthfeed/what-makes-your-skin-prone-to-acne-top-5-causes-of-acne-14523/post
"Teen Acne: What Helps, What Hurts!" by Dr. Jolly Shah Kapadia - https://www.practo.com/healthfeed/teen-acne-what-helps-what-hurts-14525/post