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.
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.