"Man owes his growth, his energy, chiefly to that striving of the will, that conflict with difficulty, which we call Effort. 

Easy pleasant work does not make robust minds, nor does it give men a consciousness of their powers, it does not train them to endurance, to perseverance, to steady force of will, that force without which all other acquisitions avail nothing. Manual labor is a school in which men are placed to get energy of purpose and character - a vastly more important endowment than all the learning of other schools." 

This quote comes from a sermon by William Ellery Channing to a group of mechanics and laborers in the 18th century; much still applies today.