Here come real stars to fill the upper skies,
And here on earth come emulating flies,
That though they never equal stars in size,
(And they were never really stars at heart)
Achieve at times a very star-like start.
Only, of course, they can't sustain the part.
Childhood 
 Playing off of the similarity between the words “star” and “start,” Frost implies that people who are still young (in the  start  of their lives) are more likely to directly link fireflies and stars. 
 As children grow up, they experience a drastic transition, passing from a world where everything seems miraculous to one in which fireflies represent no more than mere bugs. 
 Far too often, imagination and curiosity and wonder, and teenagers and adults no longer “reach for the stars.” 
 
 As Pablo Picasso supposedly claimed, 
 All children are born artists. The problem is to remain an artist as we grow up. 
 Though childhood doesn’t last, exploring one’s surroundings with imagination, curiosity, and wonder can always (and perhaps especially in adulthood) help break up the monotony of day-to-day life. 

Fireflies in the Garden