My guess is that either 1) Kerry is so self-absorbed that he has no sense of how this makes him look, 2) He wants to take the whole 2004 election cycle down in flames with him, or 3) The Clintons and McAuliffe are telling him to stay in the race and make things hard on Dean, even though he has no chance of winning.
CNN article here:
http://www.cnn.com/2003/ALLPOLITICS/11/14/mgrind.day.hillary/index.html
I hate to go negative on a Democrat, but this is just depressing. Of course, he and his wife have hundreds of millions of dollars in the bank, and this is basically just a hobby for him. No real stakes here for his family.