Answers like “No problem” and “No big deal” devalue the favor you’re being thanked for. They convey at once that you didn’t do very much work (even if you did). They also leave in doubt whether the person you’ve helped merits a large favor.
Saying “You’re welcome” acknowledges that you’ve done something meaningful and that you were glad to help.
In purely transactional terms, “No problem” devalues the currency that you might be hoping to deposit in your “favor bank” with the person, whereas “You’re welcome” preserves the value.
Sometimes this is about wanting to keep the option to extract a return favor. Other times it’s about preserving your time. Imagine if you get an emergency Friday night request from your boss and your response is “No problem!” Now you’ve sent the message that it’s “no big deal” for your boss to destroy your weekend.
In more human terms, saying “you’re welcome” conveys warmth and acknowledges what the person means to you (whether they’re the person who raised you from a slimy toddler into the splendid person you are, or simply the nice boss who signs your paycheck and tries not to ruin your weekends).
I admit that I don’t use “You’re welcome” very much in speech. But I’ve tried to switch to “Sure thing!” or “Of course!” These don’t make me feel overly formal, and they don’t take away from whatever small miracle I’ve performed.
