This old geezer would love to be enlightened, because the "reasons" I've read on a quick Google search have nothing to do with my reasoning. They're somebody trying to guess their way into my brain, and they are nowhere close...at least the stuff I've read.
I say "Thank you" because I really appreciate whatever the person did for me -- even if it is a minor thing. It's not a meaningless gesture; I was taught that it was just good manners in many situations. And I was also taught that the good manners response was like "You're welcome," or something similar. But I'm not "offended" by "no problem."
The main difference I see between "My Pleasure" and other more formal acknowledgments is the venue. I expect "no problem" at McDonalds, and am pleasantly surprised by "My pleasure" at Chick-fil-A. I expect "My pleasure" at Seasons 52 because it's a bit higher class establishment.