We're congratulating you because of everything you've achieved. 3000+ posts is a nice achievement for some, but it's much better when you have countless quality posts that help countless of users every day.
You thank us for the opportunity to share your experience and knowledge, we do the same in congratulating you to thank you for being with us, sharing your experience & knowledge and most important your time.
I don't believe I have congratulated you before for everything you've achieved, so I do it now :thumbsup2: