I tied between season 6 and 7 I think she developed feelings In s5/6 but didn't come to appreciate and accept love for spike until season 7 In season 6 she admitted spike was only person she could stand being around that why don't buy it simple as she was just using spike in s6

Buffy's relationship with Spike was abusive. She admitted it herself that she was just using him and didn't love him - that's why she ended it. She could only love him once he had a soul.

^ yeah but she clearly cared for him, otherwise she not trust him with dawn or got upset when he slept with anya etc, she could easily used someone else she also always turned to spike and even admitted spike was only person could stand be around and make her feel I don't think it as simple she didn't love him/using him

She said it herself in season 6 that she didn't love him at the time. That was why she wanted to stop sleeping with him. I personally think that she fell in love with him over the course of season 7, but only realised this a few moments before she told him in the finale.