In Season 7 Episode 22, why do you think when Buffy said "I love you" Spike replied "No you don't, but thanks for saying it"?

I think he said that because he wanted her to live her life because he knows that now she can because she's not the only slayer!

Because he knew she didn't love him, otherwise she would have said it sooner. She just said it to let him die happy or to give him a sense of accomplishment or warmth or to let him no he wasn't alone. Because even though she didn't love him, they were still very close.

cuz he knows buffy to well to no that she could never love him she says it to let him that she does love him but not in the way he would like

I think Buffy said it because she felt guilty. She was a douche to him a lot and used him and flat out said so, and he still died to save the world, which included Buffy. I think she loved him, but she wasn't in love with him and felt like she had to say it.

He knew she didn't love her, but that it was something between them. It was also a last minute line

He didn't believe her. I can't think of any other reason for him to say it was a lie. He just thought that she was trying to comfort him and give him what he wanted, since he was sacrificing himself. She probably did mean it though, and had only just realised too.

In my mind, I think Spike had other reasons for telling Buffy that she didn't really mean it when she said she loved him, but thanked her anyway for saying it.