As a non-American, I can confirm that I've noticed that Americans tend to develop lots of superficial friendships, but aren't very willing to open up, which keeps the relationship at arm's length. I remember I was really surprised when a coworker was telling me about how she had some condition, and then immediately apologized for telling me. I didn't understand why someone would apologize for opening up, but in the US that's considered oversharing, and is frowned upon.