Note to Hamlet, 3.2.198-199: "Where joy most revels, grief doth most lament; / Grief joys, joy grieves, on slender accident."

Where joy most revels, grief doth most lament; / Grief joys, joy grieves, on slender accident.: on the same occasion that joy celebrates the most, grief laments the most; [but] grief turns to joy, and joy to grief, on the slightest change in fortune. For an example of the kind of thing the Player King is talking about, think of a championship sporting event which is won (and lost) by one goal in overtime.