Now, this is something that only a person from the US will completely understand. While Americans are often generalized as being open and friendly, there are definitely certain subjects that are off-limits and that they would prefer not to talk about. They may go on for hours about their favorite sports team or their hometown, but ask them how much money they make or their age, and their lips are sealed.
It can be hard to understand what is taboo and what is acceptable if you did not grow up in the US.