Why does your protagonist shy away from 
military men?
Why does your hero think all beautiful women are snobs?
Why is your antagonist determined to destroy the protagonist’s career?
The answers to these questions are motivations for your characters’ behavior. It’s what drives them to do things they otherwise wouldn’t do—often against common sense—therefore, we need to offer our readers reasons for this unreasonable behavior.
When writing my first novel, I needed my protagonist to go to a public event where there was a possibility that she might run into someone who could recognize her for who she was. Since she is an intelligence agent, however, this ran counter to all of her training and experience. After each change I made to the story, my husband read the scene for me, and each time, he asked me the same question, “Why would she do that?” I hadn’t provided sufficient motivation for him, as a reader, to understand without question why she behaved this way. It would be a spoiler if I told you how I finally changed the story, but suffice it to say that I created a circumstance sufficiently dire that it would make her take the risk. (If you’re interested in finding out what I did, check out my novel, Peril, in the Castle Gate Press bookstore)
Sure, you can say that it’s just fiction, so why is such depth of motivation needed? Like my husband, many readers will sense that something is wrong when a character’s motivation isn’t strong enough to explain the behavior. It breaks the story’s spell and makes it more difficult for the readers to set aside disbelief. While it might not be enough to make every reader quit reading, it still creates a disconnect, possibly enough that the next time they see one of your novels, they might pass it up for a different book.
So ask yourself the tough questions, don’t skim over them. Don’t let your characters off the hook. Make them dig into their past, create difficult circumstances for them, give them something to fight for or overcome. The readers will not only be hooked on your story, but will happily allow themselves to be reeled in so they can find out what happens.