Be first to read all about What Acne Is to understand how it involves clogged pores. Once you understand that the pores are getting clogged up, it's time to learn just WHY this whole thing happens to some people and not to others.
First, we all know that acne tends to strike teenagers. Why is this? It has to do with hormones. Hormones are released into the body starting around the teen years, to help a "child" body mature into an "adult" body. Hormones are the things that make a female grow breasts, a man's voice grow deeper, and so on. One set of hormones - the androgens - causes oil production to increase. It makes sure that as the adult body gets more active, it can stay cool.
Once a person gets into their teens and the oil production really starts to chug along, you might think that every single adult would have acne for their entire lives. However, this isn't the case. The human body tends to equalize and become able to cope with this situation. However, certain situations tend to cause problems. For example, when females have their periods, their hormone levels go up and down. Some women find that they have acne flare-ups when this happens, because the hormones cause a sudden abundance of oil - and some of those pores get jammed up.