1. She loves him. She knows she does. At the end of Dead Things when Tara asked her if she did .... did she say anything? Nope, which is a good thing for us B/S shippers ;) Because when Someone would normaly ask her about Spike, like if she loved him, she would have normally yelled out NO! right away.
2. He would do ANYTHING for her. She is held so high in his eyes, and she knows this. If she was sick he would take care of her. If she needed help he would definetley help her. She could depend on him to do anything for her.
3. He Loves her. They could have such a good relationship together, if Buffy tried to to actually show love to him.
So these are three reasons why Buffy should actually try to show love to Spike.