It is a quote by Albert Camus.
I didn't know that either, although I knew it certainly wasn't 'happy-clappy' in origin as fiverings suggests. Also it doesn't seem in the least banal to me because it's so wise, and I love that pic because it speaks volumes about the meaning of true friendship.
You do realise though Sabine that once again I've gone all teary-eyed. At this rate we'll be emotional wrecks by the time Montreal comes!