motive

US /ˈmoʊ.t̬ɪv/
UK /ˈmoʊ.t̬ɪv/
"motive" picture
1.

a reason for doing something, especially one that is hidden or not obvious

:
The police are still investigating the killer's motive.
What was her motive for lying?
1.

to provide with a motive; to impel

:
His desire for revenge motivated him to act.
The promise of a bonus motivated the team to work harder.