Science has confirmed that one way to get your readers to enjoy your stories more is to make sure there are some surprises along the way. In an article adapted from "100 Things Every Presenter Needs to Know About People," Susan Weinschenk describes the research:
"(Researcher Gregory) Berns used a computer-controlled device to squirt either water or fruit juice into people’s mouths while their brains were being scanned by an fMRI device. Sometimes the participants could predict when they were going to get a squirt, but other times it was unpredictable.
The researchers thought that they would see activity based on what people liked. For example, if a participant liked juice, then there would be activity in the nucleus accumbens, the part of the brain that is active when people experience pleasurable events.
However, that’s not what happened. The nucleus accumbens was most active when the squirt was unexpected. It was the surprise that showed activity, not the preferred liquid."
Of course the challenge is making sure that the surprises actually make sense. For instance, if you have a character who seems like a nice person betray his best friend, plant some clues beforehand that will help us accept he'd do such a thing.
Maybe he is deeply in debt and the only way out is to sell some private information about his friend to a tabloid newspaper.
Or perhaps he is taking medication that is having an effect on his mental state and we see some examples of that before.
The nature of the motivation will depend on the character you have created, but the important thing is to plant a few clues so subtle the reader won't attach any particular importance to them at the time but will recognize them as clues after the fact.
One thing I found surprising in the story above is how scientists pitch their research ideas with a straight face: "Ok, well, we're going to spend the grant on squirting water or orange juice into people's mouths..."
(The great writers knew about the power of surprise all along. You can find what writers like Dickens and Twain and Austen advised about writing by getting a copy of my book, "Your Creative Writing Masterclass." In fact, why not buy it for that special person in your life-- getting it will be a nice surprise. Less messy than squirting orange juice into their mouth.)