Acne can occur frequently among teens and it is a result of the hormonal changes that come with puberty. They can also be genetic, if your parents had them, it is likely you will too. For most people, acne disappears on its own by the time they reach their twenties.
The pores in your skin have sebaceous glands that produce sebum, which is an oil that lubricates your skin. During the teen years, sebum is produced excessively, resulting in the pores becoming clogged. Bacteria gets trapped inside of the pores and start to accumulate, causing acne.
If you see acne or pimples in the mirror, it is best not to touch it. Picking at them can further increase inflammation or leave permanent scars on your face.