Angelus liked watching Buffy. He hated her, loved her, wanted to fuck her. She was his obsession, even more interesting than Dru, and he just couldn't quit.
He lurked around the edges of her life always making his presence know, always felt. He hunted her schoolmates, sent her love poems, killed her teachers, but never once had he threatened those she loved. He had amnesty so long as he didn't cross that line. She would take anything else he threw at her. Protection from the Slayer was every fledgling's dream, and now he had it.
It kinda turned him on.