Why are the Kennedys so popular?

Why is it that the Kennedys have always been so popular. I don't know much about what they did or who they were, but they've always been treated and remembered as celebrities in the political world. I know JFK was president killed my Lee Harvey Oswald and that one of the Kennedys banged Marilyn Monroe, but I haven't heard of anything that they've actually done. Did they actually do something to deserve all this attention, or were they basically like Justin Beiber: famous for no reason?