He believed in 2008 that he genuinely had a chance. It was kind of a off-chance, but think about this: After 8 years of Bush, people were fed up with that line of thinking. However, an anti-war, lower taxes candidate could have knocked the wind out of the democrat's sail as well, since their anti-Bush arguments won't work against Paul. But republicans were idiots and didn't give him a real chance. McCain and Palin instead.
Paul might have gone with, who knows, Kucinich as the VP candidate, further damaging democrats chances.
This time, it is probably more to put forth the libertarian agenda for the people. The rest of the GOP candidates are scary bad.