When, four years old and sitting on the stairs, I was tying my shoelaces, I always got entangled in this problem – basically, a confict of symmetry and balance:

when I put on my left shoe first, then the right,

should I then first tie the left, or the right?

Left first: then L-R, L-R ; right first: L-R, R-L;

in both cases: twice the same thing (at different levels), that is, a disruption of alternation!

A tonal sentence with two alternating chords (I and V) can only be tied up by a disruption of balance (an extra bar: I V | I V | I) or by disrupting the regular alternation (I V | I-V I).

Passamezzo (a pass-and-a-half): I IV | I V | I IV | I-V I).

That is: the shuffling origin of the cadence?