The main reason why I believe in a conspiracy to at least some degree is that there are just too many freak coincidences. David Ferrie's connections to Oswald and Carlos Marcello, Oswald's uncle working for Marcello, Oswald's connection to the CIA, Jack Ruby's connections to Marcello, Oswald's connections to anti-Castro groups and their connections to the CIA and Marcello, etc. Most of these connections are established facts and simply cannot be ignored.
This is exactly why I have always believed in a conspiracy.