So much can be read into this interaction; Spike, not having a conscience, is confused as to why Buffy would turn herself into the police, considering Katrina's 'death' was a mistake. Buffy doesn't understand why Spike doesn't understand that's not the point. It's that she killed someone - regardless of if it was a mistake or not. Spike sees things differently than her, and for that she punishes him. All he's trying to do is protect her from ruining her life (and protect Dawn). When Spike asks her to explain it, he's honestly seeking answer. He truly doesn't understand why this is "killing her." To understand, he needs a soul. So he gets one. So he can understand her and be worthy of her.
This scene is tough, but's it is SO important to understanding their conflict.