David Muir left fans feeling emotional following his latest social media post, as he paid a heartfelt tribute following a heartbreaking loss.MORE: David Muir's heartwarming photo from home has fans gushingUpon learning of the tragic passing of writer and journalist Joan Didion, the Good Morning America star took to his Instagram stories to pay his respects.He uploaded a picture of the cover of her 2005 book The Year of Magical Thinking, and below it simply wrote: "RIP Joan Didion."WATCH: GMA's