Temperature and humidity are the main causes of pitch change. A piano's main acoustical structure, the soundboard, is made of wood. Wooden soundboards produce a wonderful sound, but they also react constantly to climate changes.
As the humidity goes up, soundboards swell, stretching the piano's strings to a higher pitch. When it is dry, especially in the winter, the soundboard flattens out, lowering tension on the strings and causing the pitch to drop.
The drop in the summer or when the heating is high in the winter tends to exceed the rise during cold humid times. As a result pianos tend to fall more out of pitch each year that they are not serviced.
Regular tunings, ideally every 6 months or each season will guarantee a consistent sound and will help avoid costlier repairs in string replacement or repairs to the various wooden parts of the piano.