They are internal - Characters react to things that touch something in their lives and pasts.
They can be rationalizations. Just what is a rationalization? This is the reason a character gives for the way they react and act to a certain situation. Say when they were a child, a dog bit them. Any time they see a dog they freeze. They tell themselves this is because of the dog bite that exists in their memories.
They often contain secrets the character fear will be revealed. Secrets a character has inside like fearing everyone will know they're a coward, or some other thing will strengthen the character's motivations.
They are often layered. This means there are a number of incidents that lie in the character's reasons for acting or reacting in a certain way.
They will goad the character to take action.
Not all a character's motivations will contain all the traits but they must contain at least one and probably more.