[The Scientist (OP)]

Is it because we touch our face with our dirty hands? Consume oily food? Or what else?
What do you think? Thanks!

[CliffordK]

Certainly toddlers touch EVERYTHING with their hands, yet have low incidence of acne.  And, a teenager washing their face may help...  but won't cure it.

There seems to be multiple issues. 

Hormonal Changes in Puberty
Bacterial infections.
Immune Response.
Susceptibility to certain types of acne which may even be hereditary.
There are also indications that GI tract disorders may be linked to some types of acne (although only accounting for a small subset of the acne).