http://www.slate.com/articles/news_and_politics/politics/2015/10/joe_biden_needs_to_answer_this_question_if_he_s_going_to_run_for_president.html

Joe Biden is expected to announce whether he is running for president any day now. If he does decide to jump in the race, there will be one critical question for which he doesn’t quite have an answer: Why are you running? Right now, there are many reasons and no reason at all: He has always wanted to run (and has run unsuccessfully twice). His son Beau, as he was dying, encouraged him to run. Hillary Clinton is not doing well, and some in the Democratic Party think he is a better general election alternative. He’s not yet ready to see the end of a long career. He connects well with middle-class voters. He has experience.