Humans want to believe certain things about their world and about themselves. We often know these things aren’t completely true but we’d like to think so. For example, we’d like to believe that good triumphs over evil. We’d like to think the little guy can beat the big guy. We’d like to know that justice will be done. We’d like to think there is someone for everyone. We’d like to think that our special qualities will be recognized and appreciated.
These beliefs are important to us, and we seek out fictions that reinforce them. We watch movies where the good guys win. We read novels where true love triumphs. We root for the underdog. Just consider movies like Raiders of the Lost Ark, Pretty Woman, and Rocky. Or The Matrix, Bridget Jones’s Diary, and Rudy. Or many, many others. Consider books such as Misery, The Odyssey, and The Lord of the Rings.
We writers have the same kinds of beliefs, and I’ll bet if you analyze your own writing you’ll see these themes showing up in your work. Of course, not everyone wants exactly the same things from their fiction. Nor do they want them presented in exactly the same way. But it’s rather hard to go wrong with good guys and underdogs winning, and with characters finding love.
It's also hard to go wrong with characters who discover something special about or within themselves, as in the Harry Potter and Twilight series. It's hard to go wrong when you give people an insider's view of a profession or a lifestyle they find fascinating.
In other words, it's hard to go wrong if you really find out what readers want to believe and experience and give them fictions that reinforce those beliefs.
Think about it.