Mainspring

[Sarah]

The principal or most important spring in a piece of mechanism,especially the moving spring of a watch or clock or the spring in agunlock which impels the hammer. Hence: The chief or most powerfulmotive; the efficient cause of action.

[Rihanna]

the most important reason for something; the thing that makes something else happen:

[Couch Potatoe]

Something that plays a principal part in motivating or maintaining a movement, process, or activity.

[Donald Trump]

The main agent of motivation.