It's impossible for any one person to know and understand himself completely. Certainly there are different levels of self knowledge but no one ever attains complete knowledge of his or her psyche. Whenever I start to think I know myself as well as I possibly can something happens to prove otherwise.
I might become really upset about something that doesn't seem like a big deal. I find myself asking, "Why did I get so upset about that?" I might really dislike a particular person even though she has never done anything to me. I'll wonder, "Why don't I like this person? What is it about her that rubs me the wrong way?" Maybe I'll feel sad or angry for no identifiable reason. I'll ask myself, "What's bothering me? Why am I in such a bad mood?"
It's possible that I won't be able to answer these questions, no matter how long I dig for them. Some things are so deeply hidden somewhere in our unconscious minds that we'll never unearth them. And so there is always a part of oneself that remains a mystery, both to you and to everyone else.
If a person remains a mystery to himself then he will never come close to completely knowing and understanding someone else. Yet we make the mistake all the time of assuming we know everything there is to know about those closest to us. Most of the time it is a spouse or romantic partner who we believe no longer holds any surprises for us. After years of being in a committed relationship with the same person, many of us grow bored with the monotony of daily life. We complain that there is no longer any passion in the relationship. Life with this partner has become too predictable; there is no longer any excitement. We might wistfully recall the early days in the relationship when just seeing our partner gave us butterflies. We can't pinpoint exactly what's missing in the relationship; it just seems like we know everything there is to know about our partner and we've become bored. This happens all the time; it's one of the more common reasons for relationship dissatisfaction and even divorce.
In reality, the problem isn't that we know all there is to know about our partners; the problem is that we think we know everything there is to know so we stop being curious. We assume we know the reasons for their actions (or lack thereof); we believe we know how they feel and what they think. We're wrong. In fact, a 2009 study by Tsapelas, Aron, and Orbuch (http://data.psych.udel.edu/laurenceau/PSY467Intimate%20Relationships%20Spring%202010/Readings/aron-marital-bordeom-2009.pdf) suggests that believing we know all there is to know about someone actually creates distance in a relationship. In other words, the more we think we know a person the less we probably do.
The point is, it's impossible to fully know and understand another person, anymore than we can completely know and understand ourselves. We are cheating ourselves when we make the assumption that we know everything about someone. We enrich our lives when we remain curious and continue to ask questions; the result is deeper and more fulfilling relationships.