Ask yourself where are they coming from. Explore their back-story and find the moments that changed them for ever: those moments we all have and never forget that make us the people we are and therefore affect our behavior. For example: the day her grandmother died and Natalie realized no one would understand her like that again. Or the day John realized he had to make something of himself so that his mother would respect him.
Abraham Maslow identified a pyramid-shaped hierarchy of human needs that also provide useful signposts for motivating your characters. According to his theory, at the most basic level, humans are looking to satisfy their fundamental physiological needs (for example they want food and warmth) before they start working to achieve safety and stability, then search for love and intimacy. Only when those things have been achieved can they go after the things which will bring a sense of self-worth and self-knowledge. Exploring these needs in relation to your characters can give you clues about what your people really want and need at any given point in the story and you can construct your narrative accordingly.
What are the virtues and the vices of your characters? Does that give you a clue to what drives them and what might get in their way?
Run a plausibility-check over motivations. Is it too much? Is that credible? Would a person really do that? Again a rich, well thought out back-story may help give you the answers.