One of the principles of writing that I aspire to adhere to is that we live in a world of transaction. That is, everything costs something. Nothing comes for free.
Understanding transaction isn't the same as cynicism. Sure, love costs something. It exacts a price in loyalty, energy, and time. We make sacrifices for love: we put up with our parents even though they annoy us, we spend time with our children even when we're tired, we agree to only have sex with our spouse even when we're tempted. However, the price is right. We make these sacrifices gladly.
In part, it's how we think. The best things in life have value in part because they cost so much. We know they're worth something because we have to fight to get them and sacrifice to keep them.
Fiction - especially fantasy and science fiction - is often wish fulfillment. We read to vicariously experience lives other than our own. Sometimes the wish fulfillment is obvious (who doesn't wish he had magic powers, a sweeping destiny, and a magic dog?). Sometimes it's more subtle - even though you probably don't wish you were a gay cowboy in a doomed love affair you're still reading for the passion and the tragedy - but it's always present.
However, wish fulfillment can go too far. When worthwhile things stop having a cost, the story is cheapened. We want to read about good but flawed people achieving great things at great (and stirringly dramatic) cost, not good but flawed people achieving great things with little or no difficulty. Or alternately, we want to read about good but flawed people ruining themselves because the cost of their desires is too high and they fail to pay... or they pay anyway, and are destroyed. That is the importance of transaction. Think about it. Would anyone want to read Of Mice and Men if George hadn't... I won't spoil it for you. Let's just say that peace has a price, and it is paid in the end.
In fantastic writing the importance of transaction can be even more striking thanks to the presence of magic (or technology, or whatever). The magic or magical technology of science fiction and fantasy exists to grant wishes. It is in the consequences of these wishes that fantastic literature finds its plots, its dramas, it's burning zeppelin experiences. However, if too many wishes are granted too easily, happiness becomes too cheap and the story loses its drama.
This is one of my (several) problems with Stephanie Meyers's Twilight series. In this world, being a vampire is too easy. They are beautiful, powerful, and immortal... at the cost of being too sparkly to come out by day without the harsh light of the sun revealing their nature. I much prefer Anne Rice's vampires, who drink blood to live, must contend with the gradual cooling of their human emotions, and die from sunlight. Call me a traditionalist, but I want beauty, power, and immortality to cost something.
Actually, call me a transactionalist.
Whenever I write, I look for the costs. My characters go after what they want. Along the way, they find smaller goals and pay smaller prices, all leading up to the realization of what their real goals, their heart's desires, are going to cost. Then, they pay the price, or try to cheat, or give up and go home. The story happens in the moments along the way, the small decisions, the little victories and tragedies. It may sound formulaic, but it really isn't. It's just life in a world of transaction.