I've been practicing saying "you're welcome" more often, because I noticed that some people would honestly be confused, suspicious, or offended if I said "no problem" after doing something that was obviously a bother.

For example, I'm stranded at 3am and you give me a lift even though you were sleeping. If you say "No problem", since it's obviously a lie, I think that you might be mad or disappointed that I made such a request.