Fantastic Ted talk by Paula Scher.
She differentiates between Serious and Solemn work and then goes on to outline the conditions which are present when she produces her best (Serious) work.
She explains that she is usually underqualified for the job and involved in serious play.
When this work becomes perfected (therefore Solemn) - and copied by other designers!- it's time to move on to a new challenge.
I love her chart above which outlines this creative pattern.