motive, motif

Motive or motive means the thing that prompts someone to a particular action, as an incentive or as a driving force--
The motive was revenge.
He had the means and the opportunity to commit this crime, but what was the motive?

Motif or motif means a recurring theme or or idea in a work of art--
The music changes tempo whenever we hear the horn motif.
His favorite literary motif is falling into the water.

Now that you know that, you can say--
"The motive behind his doodling was to develop a motif he could use in his paintings, as a signature."