What do you say when someone tells you "Thank you" when you're an native English-speaking American?
Well, chances are, it's not "You're welcome." Despite what your parents (supposedly) taught you.
Far more common and more popular choices include, "Sure," "No problem," and the all-purpose "Mmm."
Well, that's because, in the US (Land of the Free, Home of the Brave, and all that), people do not like being indebted to each other. As such, it's a social no-no to make people feel indebted to you unnecessarily, and saying "You're welcome" to someone is a tacit way of acknowledging that, "Yes, indeed, I did inconvenience myself for you." The alternatives, such as "No problem," have a different implication--they literally mean that it was nothing and that, therefore, there is no debt owed.
Interestingly, the most common uses of "You're welcome" these days are either ironic...or it is used by a person who is not thanked but believes he SHOULD be. (As in--Person #1
: "Uhh...you're welcome?" [Read: Hello!? Hey, I just went out of my way for you. Some acknowledgment would be nice.] Person #2
: "Oh, right! Thanks!" [Read: Yes, I am grateful for all your help, and I'm sorry I didn't acknowledge it earlier.]