Season 5. 'Nuff said. What with the kiss in Intervention, and the treating-like-a-man...

Once more, with feeling.

i'm not sure really, i've always thought she started falling for him in season 4 because she could of easily killed him what with the chip and all, but she didn't

watch the episode in season 4 called Superstar, due to a spell Buffy does not have her usual defensiveness and guard up. Look what happens when Spike gets a little, uh, close to her when her normal defences are not present when he runs his hand over her hair, touches her cheek and then brings it down towards her chest. Watch what she does:

you see something, just a glimpse of something (sexual attraction and infatuation with Spike) which she obviously hides usually under tonnes and tonnes of denial and brick walls of defensiveness.

IMO her feelings for him have been there for a while and have LONG been hidden. She fell in love with him when she no longer had an excuse not to (no soul) I think she actually allowed herself to love him in Showtime season 7. She allowed herself to have feelings for him in Intervention season 5