The reason a character thinks feels or acts a specific way is called

The way in which a character thinks, feels, and acts in a specified way is called character motivation.

The character motivation plays an important part in a story telling or novel writing. This tell the reason behind the way the character acts under the given situation.

A writer can give character motivation as two types, external motivation and internal motivation. The external motivation is includes physiological needs (food/water) and safety needs (save world from evil clutches). The internal motivation includes belongingness needs (finding love), esteem needs (prestige), and self-actualization (overcoming bad habits). A writer can use Maslow’s Hierarchy of Needs as their key to build a character.

If this is not clear, the story leads to dissatisfaction of the reader.

A reader should understand the character’s motivation to get a clear picture of the story and to enter into the character's head.