As a general rule, when you are a vampire slayer... you shouldn't fall in love with a vampire. Not least because it might affect the dynamic of your TV show and have your viewers turning off. But fall for Angel, Buffy did and it was something special! Great drama always involves tragedy and this was no exception. After they finally consummated their relationship it invoked a curse on poor Angel and turned him into a sadistic murderer. The only way to ensure the safety of the world was that they could never be together again, to the dismay of fans worldwide.