In terms of D&D:
...when you get a critical on the BBEG twice in a row.
...when you see the look on that power gamer's face when he gets a dose of his own medicine.
...when everything in your campaign goes right all the time, everytime.
In terms of real world:
...finding true love, in this world and the afterlife.
...finding a cure for cancer, infectious diseases, and psychological disorders.
...ending homelessness for children.
...finding a solution for everything.