In literature, the reason why a character says or does something is called a motive.
The definition of motive is something that causes or leads a person to act in a certain way, do a certain thing. So, in accordance with this definition, any action a character takes, or any decision he or she makes thorough the whole story, which will lead to other events wich will develop the entire story until the end, is known as motive.
Some examples of motives could be: revenge, love and the like.