It is an old and tired cliché; an actor demanding of the director ‘what is their motivation?’ The actor is asking the wrong person, but sadly in filmmaking the screenwriter is too often shunted to the end of the production priority queue.
However all writers, screenplay and prose, should take this question seriously and ask it of themselves repeatedly. You see what the actor in the cliche is seeking is their character’s goal, usually one that is bound to the scene in question.
For a scene to be about something, someone has to want something and they need to either get it, or fail to get it. The consequences of that want and its outcome drives the action of following scenes and the story. If an actor is asking about their motivation then their character is likely lacking a clear goal and the scene may be nothing more than exposition.
If as a writer you find that it is difficult to write a scene, if it seems to lie lifeless on the page, perhaps you have neglected to give the character a need and something that is denying the fulfilling of their desire. Look at the scene, the characters, and ask the critical questions of storytelling.
What do they want?
What’s stopping them from having it?
How far will they go to get it?
Keep this in mind, know the answers cold, and your characters will drive the story forward.