whats the reason people get acne?!

Hormones
Family genes
Hygiene
Diet
Clogged pores
Some cosmetics
Irritated, over-dried, or excessively oily skin
Pollution in the air
Not having a good skin care routine
Not changing pillow-cases and sheets
Dirty or oily hair
Touching your face
Some laundry detergents if the person has sensitive skin

Many, many, many things. Its hard to cure because everyone is different.

Excessive sebum (oil) and bacteria. The bacteria attaches to the skin, the excessive oil reaches the surface, they meet and form a pimple or many smaller ones.

Stress can cause excessive sebum production. So can a poor diet, hormones (why teenagers get it and pregnant women do too). Some people just have naturally oily skin and will into their old age (normally mature skin tends to get quite dry).
Being exposed to a dirty air will also risk more bacteria getting on the face. Touching the face, however, is far more likely cause of acne spreading of the face, as people tend to pick at their face or use their hands as headrests. Picking at pimples tends to spread the infection, making it more likely another pimple will form next to the picked one.

I've worked in health and beauty for 4 years.

Dirt gets into the pores of the skin and clogs it up. In order for your skin to clean it up it creates a zit, pimple, what have you. This is just the skins way of cleaning the dirt out. Kind of weird, but that's just how the human body is...Weird.

My brother-in law is a dermatologist. He says it's usually a bacterial infection. Hormones constantly changing the condition of the skin just make it easier for the infection to take hold.

Stress.

I has acne :/