Why do people get tan in summer?

To improve their appearance and health.

Appearance:

Unhealthy people often appear pale. Healthy people often appear tan/rosey in color. Therefore, a healthy tan makes a person appear healthier and therefore more attractive to people they would like to attract.

A natural tan is what most foundation makeup & tanning products try to emulate.

A healthy natural tan also helps to make some facial features stand out better (i.e. the whites of eyes look whiter--which helps to enhance eye-color, teeth appear whiter, lips often appear to have more color.

Health:

Tanning properly increases the production of vitamin D (which is good for strong bones & teeth).