How many people really know you?

A friend asked this question over the weekend ... as people started to answer, I realized that while some people know very well how I would respond in many situations, I don't think anyone really knows why I respond how I do. Sadly, the same is true for me too - I don't really know why my best friend responds the way he does, and my guess is because we tend to imprint our reasoning onto others decisions. For my best friend, we share numerous similarities (we've both been programmers from a very early age, etc.) to where we more often than not think along the same lines, but still don't know the "why" about each other ... imagine trying to truly know someone of the opposite sex!
So, how do we really get to know someone? Sure, learn more about their past ... What are questions to get at the "why"? How can we push our own explanation for others actions aside? Is it as simple as asking "Why did you do that?" Possibly paying closer attention to emotions ...