About the "deep" people, whatever that means

Just a late night thought. What makes a person "deep"? What exactly do we mean when we refer to somebody in this way? Are we not all made in one image?

Some people seem to mean intelligence, some extreme "spirituality" (personally, I don't like that idea, but that's a whole different topic). Sometimes I wonder if we are all of equal "depth". And if not, what makes the difference?

I found that the more I get to know some people, the deeper they seem to get. It's a bothersome conclusion: means that with the shallow people I just don't take the time and make no effort to dig. Reminds me of the fairy-tales, where the true treasures are often hidden where only few will look.