When public opinion is particularly polarized, as it was following the end of the Franco regime, monarchs can rise above " mere" politics and " embody" a spirit of national unity.
By contrast the recent happy photographs of four generations of the Royal Family together suggest the sort of continuities the frenzy of modern politics can never supply.
Well I guess his looks, his acting ability, his short life, and maybe the type of character he played in his movies. Many young people saw him as a symbol of the American youths.