I thought because he told Spike in 'A New Man' (iirc) that he might have a higher purpose, which S4!Spike rejected, and whether or not he then adopted (*flashbacks of old btvs wanks over Spike's redemption or lack thereof and it's motivations*) said higher purpose; Giles pretty much had given up faith in him, by then. (See 'Lies My Parents Told Me', etc.)

Or it could be a one-line joke like shikishi suggests, in which case I am a prat.