Working earnestly means being diligent and always assuming a sincere attitude toward work.
It is from work that we derive the kind of happiness to truly savor. If we neglect work and try to obtain this pleasure from a hobby or recreation, we may enjoy it temporarily, but we won't achieve true happiness.
Unless we derive a sense of fulfillment from our work - which comprises the greater part of our lives - we'll eventually feel unsatisfied.
It's only when we've accomplished something, after concentrating on our work diligently and earnestly, that we experience unfettered happiness.