Keep a green tree in your heart and perhaps a singing bird will come.
-  Chinese proverb

Exuberance is Beauty.
-   William Blake

The clearest sign of wisdom is continued cheerfulness.
-  Michel Montaigne

"Cheerfulness removes the rust from the mind, lubricates our inward machinery, and
enables us to do our work with fewer creaks and groans.  If people were universally
cheerful, there wouldn't be half the quarreling or a tenth part of the wickedness there is. 
Cheerfulness, too, promotes health and morality. 
Cheerful people live longest here on earth, afterward in our hearts."

O happiness! our being's end and aim!
Good, pleasure, ease, content! whate'er thy name:
That something still which prompts the eternal sigh,
For which we bear to live, or dare to die.
-  Alexander Pope

The best way to cheer yourself up
Is to cheer someone else up.
   Mark Twain

The man who radiates good cheer, who makes life happier wherever he
meets it, is always a man of vision and faith.
  Ella Wilcox

Happiness comes of the capacity to feel deeply,
to enjoy simply, to think freely, to risk life, to be needed.
   Storm Jameson