So ok the there a lot reasons why Spike wants to spy,steal Buffy's things. I mean couldn't Spike get over her. Its sick and twisted. But the epsiode "Intervention" in season 5 he had Warren build a Buffybot just to make it love Spike. Ok ew, but when Buffy figure out wants going on she forgave Spike because Spike was beatened badly by Glory, to find information on Dawn aka The Key. She gave Spike a thank you kiss. But in the end of the episode she told him she will never forget and that was really what he did for her to save her sister's life.