Dr. Seuss: Trending quotes

Variant: You know you're in love when you can't fall asleep because reality is finally better than your dreams.

Often attributed to Dr. Seuss without citation; also cited as an anonymous proverb.
This quote has also been attributed to Gabriel García Márquez, in Spanish: "No llores porque ya se terminó, sonríe porque sucedió."
Compare lines from In Memoriam A.H.H. of Tennyson:
  'Tis better to have loved and lost
  Than never to have loved at all.
Disputed
Variant: Don't cry because it's over, smile because it happened.

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

Source: I Can Read With My Eyes Shut! (1978)

Source: The Lorax

Variant: Today is your day, your mountain is waiting. So get on your way.

I Can Read With My Eyes Shut! (1978)

Source: The Lorax (1972)

Bernard Baruch in response to a question by Igor Cassini as to how he handled the seating arrangements at his dinner parties, as quoted in Shake Well Before Using: A New Collection of Impressions and Anecdotes Mostly Humorous (1948) by Bennett Cerf, p. 249; the full response was "I never bother about that. Those who matter don't mind, and those who mind don't matter." This anecdote is also quoted online at Chiasmus.com http://www.chiasmus.com/archive/msg00241.html. It has also become part of a larger expression, which has been commonly attributed to Dr. Seuss, even in print, but without citation of a specific work: "Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind."
Misattributed
Variant: Be who you are and say what you feel because those who mind don't matter and those who matter don't mind.

Horton Hears a Who! (1954)
Source: Fox in Socks

Variant: Sometimes the questions are complicated and the answers are simple.

Source: The Cat in the Hat

Variant: You can find magic
wherever you look.
Sit back and relax,
all you need is a book.
Source: The Cat in the Hat