The first crossover of the series, Buffy comes to visit to berate Angel but she ends up getting the man she always wanted when Angel is turned human. The one thing that HE'S always wanted, so both people get what they want, they can spend the rest of the lives together right?
...Wrong, because evil's still afoot in Los Angeles and Angel decides to pick fighting evil rather than spending the rest of his life with the woman he loves. The two spend the night together and Angel feels perfect happyness without turning evil. But due to the Morra demon returning, Angel's lack of strength and even more evil on it's way Angel decides that he'd be better as a Vampire.
After convincing the Powers That Be to turn back time, he let's Buffy walk out of his life and even though Angel lost the one thing he truly loved, he has the perfect day in his mind forever.