Dax Shepard sat down with Anderson Cooper at the New Orleans Book Festival on March 11 to talk about grief. More than 13 years after his father’s death, Shepard still can’t tell the story of Kristen Bell’s surprise without breaking down. In 2012, Shepard’s father, Dave Robert Shepard Sr., was dying of small-cell carcinoma in a Detroit hospital. Every weekend for months, Shepard flew from Los Angeles, where he was filming Parenthood, to take care…