“You never know what's coming for you.”

Georges Duhamel in THE HEART'S DOMAIN (1919). As it was composed in French, the wording in English may vary in translation. Theodore Geisel / Dr. Seuss was born in 1904, and would have been about 15 years old at the time that it was published. The full text can be found at the link below: We do not know the true value of our moments until they have undergone the test of memory. Like the images the photographer plunges into a golden bath, our sentiments take on color; and only then, after that recoil and that trans-figuration, do we understand their real meaning and enjoy them in all their tranquil splendor.
Misattributed

Narcissus and Goldmund (1930)

Variant: If you've never been hated by your child, you've never been a parent.

Source: Pride and Prejudice

Ch. IX : Outdoors and Indoors, p. 336; the final statement "quoted by Squire Bill Widener" as well as variants of it, are often misattributed to Roosevelt himself.
Variant: Do what you can, with what you have, where you are.
Attributed to Roosevelt in Conquering an Enemy Called Average (1996) by John L. Mason, Nugget # 8 : The Only Place to Start is Where You Are. <!-- The Military Quotation Book, Revised and Expanded: More than 1,200 of the Best Quotations About War, Leadership, Courage, Victory, and Defeat (2002) by James Charlton -->
Variant: Do what you can, with what you've got, where you are.
Context: There are many kinds of success in life worth having. It is exceedingly interesting and attractive to be a successful business man, or railroad man, or farmer, or a successful lawyer or doctor; or a writer, or a President, or a ranchman, or the colonel of a fighting regiment, or to kill grizzly bears and lions. But for unflagging interest and enjoyment, a household of children, if things go reasonably well, certainly makes all other forms of success and achievement lose their importance by comparison. It may be true that he travels farthest who travels alone; but the goal thus reached is not worth reaching. And as for a life deliberately devoted to pleasure as an end — why, the greatest happiness is the happiness that comes as a by-product of striving to do what must be done, even though sorrow is met in the doing. There is a bit of homely philosophy, quoted by Squire Bill Widener, of Widener's Valley, Virginia, which sums up one's duty in life: "Do what you can, with what you've got, where you are."

Variant: Parents aren't the people you come from. They're the people you want to be, when you grow up.
Source: Handle with Care