There's 3 different reasons
1. Genetic - you may have pimply skin (because of umm dirty blood). Sometimes it's in people's blood. Some people have them on their face/neck, or on their back, or they shoulders, or all over.
2. Bad eating habits or living dirty - if you eat stuff with lots of fat in it, or something you're allergic to, or something dirty comes in contact with your skin, or if you play with a pimple too much (this one makes it spread).
3. Puberty - pretty self-explanatory.
I had a lot of pimples on my forehead when I was 15 and going through puberty, went to a lot of dermotologists, but nothing helped. I saw one of the commercials for acne treatment products and I ordered it. I was amazed to see all my pimples disappear for good after just 2 weeks.
Proactive, that's what I used. That was during the first weeks that it was introduced to the market, back then it cost around 50 usd for the 3 lotion system. It's a lot cheaper now. If you get that and stick to the instructions, you'll have clear skin in no time.