Some quotes

For all sad words of tongue and pen, the saddest are these, 'It might have been.'
-John Greenleaf Whittier

Hearts will never be practical until they are made unbreakable.
-Wizard of Oz

The tragedy of life is not that it ends so soon, but that we wait so long to begin it.
-W. M. Lewis

Feelings are not supposed to be logical. Dangerous is the man who has rationalized his emotions.
-David Borenstein

We enjoy warmth because we have been cold. We appreciate light because we have been in darkness. By the same token, we can experience joy because we have known sadness.
-David Weatherford

There are as many nights as days, and the one is just as long as the other in the year's course. Even a happy life cannot be without a measure of darkness, and the word 'happy' would lose its meaning if it were not balanced by sadness.
-Carl Jung

Don't fear failure so much that you refuse to try new things. The saddest summary of a life contains three descriptions: could have, might have, and should have.
-Louis E. Boone

Just figured I'd copy everyone else, and post quotes too.