Why was waltzing with his father "not easy" for the narrator in "My Papa's Waltz" by Theodore Roethke?

Waltzing is "not easy" for the boy in "My Papa's Waltz" because of the fear he has of his father.

The waltz in "My Papa's Waltz" could be seen as difficult because of the size difference between father and son. The "small boy" hangs on to his father "like death."  With this height disparity, "such waltzing was not easy." However, a darker vision might exist.  Many interpretations of the poem suggest that "My Papa's Waltz" depicts an abusive relationship between the father and his son. This makes the waltz physically and emotionally difficult for the boy.

As he begins the dance, the father's state makes it difficult.  The boy notes how "the whiskey" on his papa's breath "could make a small boy dizzy."  That represents one level of difficulty in the dance.  The father's inebriated state causes him to spin the boy and have no regard for the objects around them. This is why both father and son "romped until the pans / slid from the kitchen shelf."  

The father's physical force is so strong that the boy experiences a scraped ear from his father's buckle with every step he missed.   It is as if the father is "dancing" his abuse towards the son.  The father's "battered" hand "beat time" on the little boy's head with a "palm caked hard by dirt."  The father's "waltz" is an exercise in displaying power over the boy. The emotional difficulty of this dance is confirmed with how the mother's facial expression "could not unfrown itself" as she watches the spectacle unfold.  

This waltz is difficult because the father is a towering and dominating figure. He revels in his abusive display of power.  The boy is terrified as he clings to his father's shirt.  There is little chance he enjoys this nightly tradition.  The boy's fear of his father and the pain he experiences during this dance make it emotionally and physically difficult.  Such a realization underscores how this dance "is not easy."