We are outwardly saddened by the death of Robin Williams. We've seen this before with icons like JFK and JFK Jr., Princess Diana, Michael Jackson, Farrah Fawcett and Whitney Houston.
As someone who has been in the field of grief for decades, the answer is simple. Grief is a reflection of a connection that has been lost. We grieve those we love. We grieve those will like. We even grieve those we've never met. All you need to feel is a connection to feel grief.