Counting is a primal human act. Meditators count breaths, customers count change, musicians count beats and measures, when a rocket is launched we count down, in a gym class we count up for team assignment, we count the days of a month and the years of our lives, astronomers count the stars in the sky.
Somewhere in the mists of time people found that they could count repeating units of sound that, when grouped together, make words. And thus formal poetry was born.
These sound units are what we today call syllables. Just as people articulate the flow of time by counting days, poets articulate the flow of language by counting syllables.
Along with this discovery came the understanding that some words have the same number of syllables and so they share a number in common; even if the two words have different meanings, they share the same number of syllables. This also applies to groups of words; two groups of words may have the same number of syllables and so have an underlying unity, a unity of number which can transcend a difference in meaning.
Counting syllables grounds the poet in an activity that all human beings share. Because it is an activity that all people share, counting is a kind of humbling activity, keeping the poet connected to the ordinary. When the poet counts syllables, that counting is the same counting that ordinary people do when they count whatever they are counting (change, plates, votes, etc.).
By counting syllables poets replicate a pervasive human mode of comprehending existence. I think this is a central reason why formal poetry, based on counting syllables, is so attractive to people. The regularity of counted lines instantiates, and then displays, a mechanism that is central to our humanity, something that all of us do every day numerous times. Lines that are based on counting, are, therefore, immediately accessible, resonating with this pervasive, and very human, way of interacting with existence.