Concept

Set is a powerful word because it could be used to express different forms of motion. Firstly, set could be used as to describe an object that is not moving, as in set down. The way of capturing photos could be considered as to set motion down into a still image. Moreover, the process of solidification could also be described by using the word set. As it could be a slow process, a feeling of sorrow could be sensed because of the long waiting.
On the other hand, set is also a word that expresses movement. It mostly describes the act that an object or a scene that accelerates from still to motion, such as set off and set free. Therefore the word set used for motion is more optimistic and more cheerful, which is completely opposite from the sadness felt of its use for stillness.