The motivation for a character's actions and reactions is important. This is something the reader must know and understand.
The strength of the motivation must be equal to their desire. If what they desire is something major, a weak motivation will let the story fall flat.
The motivation can be good or bad. The villains in a story may have motivations calling for revenge or for hurting other people in some way. This kind of motivation can be strong. I have a villain in one of my stories who for him has a strong motive and strong reasons for his desire to win but for the other characters this motivation means trouble. Their motivations must be as strong as his.