"I need your help."
Interesting. Don't say yes right away. Let her beg a little. Make her -
"You're the only one strong enough to protect them."
Wait, what? Slayer's mom and sister - she'd trust him with their lives, him of all - well - beings! Which had to mean something, right?
Spike wasn't sure he liked the warm fuzzy feeling spreading in his belly. Better not think about it too much. Turn on the telly. Get slayer out of his head. Too many rather disturbing things going on in there lately.
Warm feeling won't go away...