Spike hated his complete inability to say 'no' to her. It had always been an issue, created more problems than it solved really. But it was there, and it would probably always be there, and they would always end up in this bloody cycle they couldn't seem to get out of.
Truth was, no matter what the reality of the situation was, he loved her. He loved her more than he'd ever loved anyone and she made him want to be different, to try to be a better man.
He wanted to speak up, to says something, anything, but he couldn't find the words. She was having that effect on him a lot recently. Maybe there was no need for words. Maybe it was just this.