Do you know why you hired your most recent employee over the runner-up? Do you know why you bought your last pair of pajamas? Do you know what makes you happy and unhappy?
Don’t be too sure. The most important thing that social psychologists have discovered over the last 50 years is that people are very unreliable informants about why they behaved as they did, made the judgment they did, or liked or disliked something. In short, we don’t know nearly as much about what goes on in our heads as we think. In fact, for a shocking range of things, we don’t know the answer to “Why did I?” any better than an observer.
Often, there are some aspects of a person that they know themselves less than those who observe them.
In any case, I do believe that observation of large groups of people in attempts to identify common stimuli and response is the best way to understand people. The difference between what people think of themselves and what an objective observer thinks is often quite vast.
The quote is taken from here, where it is worthwhile to read the whole thing.