This dog told me that the secret to life is
A fierce, unshakable hopefulness paired with a realistic understanding of your place in the world.
(And subsequently his favorite thing is licking spaghetti off own face.)
Sounds like a good mantra to me.
A close runner up was this dog who says the secret to happiness is
a persistent, unmitigated enthusiasm without context.
Still on the search for a life mantra? For other happy animals, go here.