Europe has kept its coronavirus cases low after reopening. Measures like mask wearing have been credited for the early success. WSJ’s Margherita Stancati and Bojan Pancevski explain how sentiments toward mask wearing have changed in Europe. Photo: Claudio Furlan/Associated Press Young people are letting loose across Europe, fueling a surge in coronavirus infections that is imperiling the Continent’s hard-won gains against the pathogen. Towns along the coasts of France and Spain are grappling with outbreaks after young people piled into beach bars, ignoring social-distancing rules, and caught the virus. And thousands of young Europeans recently gathered at an illegal rave in a Berlin park, defying police efforts to shut it down. …