All writers need to understand the motivation of their characters. Strong motives produce convincing storylines; weak motives make for flimsy and unconvincing stories. Your characters’ problems and desires contribute towards their motivation; but these must be logical and believable. In fact, they should be inevitable; your characters should have no choice but to act in the way they do otherwise weaknesses and holes in the plot will be revealed and the reader will not be convinced.
Circle of Motivation