A portion of this post originally appeared in the Business Insider Today newsletter. You can sign up for Business Insider's daily newsletter here. Plus, sign up for Joe Ciolli's weekday markets newsletter, First Trade — coming soon. Maybe you've had this experience lately: You're riding high, feeling good about yet another record high in stocks, and then someone pops up on TV to kill the vibe. "It's an AI bubble," they say. "It's like 1999." This actually happened to me last week (before President ...