Once upon a time, back when I was in grad school, I decided that I was going to learn to speak a foreign language. I'd enjoyed learning Latin in high school and college, but I felt like I'd missed out never having learned to carry on a real-time conversation with actual non-English-speaking foreigners. And since there were so many foreign students in the math department where I was studying for my Ph.D. (who could help me practice speaking their native tongue) -- it was an opportunity I didn't want to waste!
So I picked French as my new language, and I bought a stack of books and tapes and obsessively studied until I got to the point where I could carry on a rudimentary conversation. Then I scheduled regular conversation outings with various grad students who were from French-speaking countries, and by the time I got the opportunity to travel to France -- to spend a month attending a special semester of Number Theory at a university in Paris -- I was already able to carry on a perfectly reasonable conversation in French, as long as the person I was talking to was fairly patient.
When I arrived in Paris, I had plenty of opportunity to practice my new skill on the people of the math department I was visiting. And it seemed like everybody was complimenting me on how well I spoke French, to which I would respond with a faux-modest "merci" and then launch into the grand epic tale of how I'd taught myself to speak French. That was one of the stories I was really, really good at telling in French. ;^)
Then towards the end of the month, the compliments started to dry up. What's worse, instead of complimenting me, the new people I would meet would actually correct me -- pointing out errors in grammar and pronunciation in the middle of a conversation!
This annoyed me at first (especially since it limited my opportunities to regale people with my favorite grand epic tale), but I felt better after I thought about it a bit. When someone arrives fresh off the plane, obviously enthusiastic about speaking the language and capable of a reasonable conversation, but is making errors in every sentence, what are you going to say? The natural response is something like "Wow, you speak French well; you're learning fast..." Whereas when I'd gotten to the point where people were bothering to correct one lone mistake in the middle of a conversation, it must have meant that the other stuff I'd said was mostly right.
Thus I concluded that criticism is often a compliment. I think this is a general lesson that applies to a lot of life circumstances.
(The humility-restoring post script to my grand epic tale: now my five-year-old son Nicolas corrects my French. He's not even lenient about it either, he's like "Mom, it's not puissant, it's puissante," and he follows up with an exaggerated eye-rolling that says "duh, Mom, where did you learn to speak French?")