Trivia: In the episode 'Untouched' Angel asks Cordelia 'Do you have any idea how hard it is to think when you've got a rebar through you?', her reply of 'Actually, I do. Benefits of a Sunnydale education' is a reference to the BTVS episode 'Lover's Walk', in which Cordy falls through a staircase and is impaled on a rebar.
Are You Now Or Have You Ever Been - S2-E2
Trivia: When Angel is outside the observatory, he is dressed in the same clothes that James Dean wore in Rebel Without a Cause (a film where there are significant scenes at the same observatory), and Judy has a bit of the Natalie Woods about her too. They are also both set in the 50s.
Blood Money - S2-E12
Trivia: When the Special Guest TV Stars come out and "rob" guests of their donations at the ball, one of the starring ladies is asked "What's the deal with making your character a lesbian? Is it for ratings?" A little jab at Willow's coming out on "Buffy the Vampire Slayer"
Trivia: Joss Whedon hints at future events in season 4 during Lindsay's song in "Dead End." It hints about the coming of Jasmine: "The sky's gonna open, People gonna pray and crawl, Gonna rain down fire, Gonna burn us all," and "Pretty as a picture, She is like a golden dream." All references to Jasmine.
Answer: Due to his epiphany, Angel basically had the 'true darkness' he worked so hard to reattain ripped away from him, like his soul was after sleeping with Buffy.