Why are Stanley cups so popular and selling for hundreds of dollars?

ViewAsk a millennial or a Gen-Zer about their go-to water cup, and chances are it's a Stanley.  

Limited Valentine's Day edition 40-ounce Stanley tumblers in pink and red flew off shelves this week at Target and Starbucks, with fans lined up as early as 3 a.m.; the cups sold out in minutes. As of today, the only available...