Your reason for doing something is your motive. Your motive to go to school every day is to learn. Or maybe the thought of your parents' anger and a run-in with the truant officer is motive enough.
The word motive comes from the Latin words motivus, meaning moving, and movere, meaning to move. Think of motive as the thing that moves you to act. You often hear of motive in the context of crime––a suspect’s fingerprints may be on the murder weapon, but a detective won’t understand the crime until he understands the motive. What made the killer act?