Assuming the world we live in has a natural order and that we take part in that order (assuming we're natural), It might not be clear exactly why I'm writing this, but there's probably a "cause." It would be weird to say that the cause of my writing this is what will happen afterward. When trying to explain why people do what they do, feel what they feel, and think what they think, it's hard to find causes, mostly because human beings sometimes seem pretty random—specially when we talk about lots of people. We're usually told that causes happen before effects, but then we reinforce the idea that behavior can be caused by its consequences.
Natural science has to do more with useful, logical explanations than with accurate, illustrating descriptions. It's not that science doesn't care about descriptions at all, ultimately, every explanation comes from a description, but scientific behavior is all about questioning every single description nature offers us through our senses. Descriptions are always valid, while explanations become void and change, all the time, with every new discovery. The moon, for instance, will always be a beautiful, silvery circle that illuminates the sky at night. The moon, nevertheless, was once made of cheese by some god, but now it is made of hard rock by some chance. Good descriptions—unlike my moon description—last forever; great explanations are, at their best, as temporary as my own life.
Describing what people do, feel, or think isn't always easy, but it surely is easier than explaining why such things happen. As we believe that in nature ordered things have knowable causes, we give order to our existence by believing that we are natural as well, and so we—and the things that we do, feel, and think—must have knowable causes also. We see other animals, we state the causes of what they do based on their own survival, and we believe that if some environmental event helps them surviving (or makes their survival happier, in our case) the animal will repeat what led to that event. Then we need a bigger explanation and we put everything we know about human behavior (and feelings and thoughts) inside the brain. But the brain isn't the end of the story. Sometimes random stuff happens in the environment and we do, feel, or think random stuff, believing that our insides have some connection with the things that happen outside.
When it comes to psychological science, it goes far beyond that; far beyond behavioral consequences and causes, far beyond behavior itself. It goes far beyond the brain, far beyond the social environment through which we can know other people's brain and imagine, through a mirror, that we have one of our own. It has to do more with scientists' behavior, with their lives, with their problems, with their solutions, with their ability to make people believe. They talk about their lifetime experience in a theoretical fashion so everything that has ever happened to them can be used to explain everything that will ever happen to everyone, but it hardly does.
We'll find more causes in places where we haven't searched yet, and we'll behave, feel, and think accordingly. But only descriptions will remain effective; descriptions of a rat pressing a lever, of a monkey "learning to speak" through a human invention (that makes our survival happier), descriptions of the brain, descriptions of how great we are. I can't quite explain it yet—and probably never will—, but descriptions about us, about what we do, what we think, or how we feel, are the only thing that reinforce our explanations—even before they happen.