He's never really understood fear.
When he was a kid - privileged little ponce that he'd been - he'd had servants to be afraid for him. Don't let Master William near the fire. Don't let Master William run in the park. Don't let Master William have any fun. Might hurt himself. Might get you the sack.
All right, so he was nervous that night at the party when Cecily - bitch!- confronted him about the bloody poem, but afraid? Not really. Never occurred to him she would turn him down, did it?
And when he saw Dru's true face for the first time, he'd felt more curiosity than terror. Never occurred to him to run for his sad little life either.
Angelus had tried to teach him fear. Like most things about the old man, the lessons hurt. Funny thing was, though, the more Angelus knocked him around, the less afraid he was of him.
Do your worst, you bloody bastard.
In the end, Angelus was reduced to threatening him with the Slayer, and all that did was arouse his interest in her.
He can't remember a single moment with either Slayer he killed when he actually thought he might lose. But even if he had, what a way to go.
There's death, there's glory, and sod all else, right? Nothing to be afraid of there.
But now, finally, after a century and counting, he gets it.
As it happens, fear looks a lot like a tiny blonde girl with green eyes and an expression of withering contempt whenever her gaze meets his.
He's so afraid of that look. Would give anything not to see it directed his way ever again.
Can't bloody live without it either.
So yes, he understands fear at last.
Turns out, it's another name for love.