I believe that the reason why Spike has, well, basically a conscious, is because of the Order of Aurelius (sp?). Now, to expand on that, I believe tha slowly as time went by Vampires basically... evolved. EXAMPLE: Along the line vampires just kept getting... better! Like first it was Darla, then Angel, then Drusilla, then... Spike. Imo, Spike is like a mutation in the vampire race. (Plus, Dru probably passed some weird thing to Spike...)