I couldn't speak for him, but if I had to guess I would say that he wasn't willing to trade liberty for security, as Ben Franklin warned us about doing.
I would say that it doesn't reflect very well on Ron Paul's judgment. Fortunately, he was in a distinct minority and the bill passed and was reauthorized so that our law enforcement and intelligence communities have the tools to protect us. I assume that you are also against the Patriot Act. What liberties are being compromised?