A spoiler is a summary of a narrative or part of a narrative which relates part of a plot not revealed at the early part of the narrative itself.(More Info here on Wikipedia)
Since part of the enjoyment of a movie or book is due to the surprise element, a spoiler could spoil some of the enjoyment one would derive from a book or a movie. So spoilers are generally not desirable.
Life is like a movie. Some of the things that happen are surreal and sometimes it takes hindsight to be able to fit the pieces of the puzzle together. When there are pleasant surprises ahead or life is proceeding serenly, I am glad there is no spoiler for life. There are times when I wouldn't mind a spoiler for life though. It would help make better choices sometimes.