Ok, my hand has been forced on this essay.
Angel leaves Buffy in Season 3 because he realises that she would never be able to have a normal life as long as he's around...sacrificing something you truly want for the sake of somebody elses happiness is a true act of love. The reasons are all there for us to hear, The Mayor explains the pain to come in 'Choices' and then Angel himself does so in 'The Prom'...as well as Joyce. Giles always knew deep down and even Buffy and Willow admit that Angel is right.
In 'I Will Remember You' Angel goes to see the oracles after the fight with the demon that restored his humanity and is told that Buffy will die if he remains human as there is only one champion in the world and instead of the forces of darkness attacking both Angel and Buffy then B would just get the lot...plus she'd have to look after Angel all the time because he'd have no powers. This is what makes that an act of love:
Woman Oracle: You are wrong. This one is willing to sacrifice every drop of human happiness and love he has ever known for another. He is not a lower being.