Just for a moment focus on your breath... in slowly, out slowly.
The same pattern repeats within each one of us and consider your pulse, the beat is built within the fabric of our being. To put it simply, we're creatures of rhythm and repetition. It's central to our experience, rhythm and repetition.
And we delight on those aspects everyday, in the rhythm of a song or the beat of the drum, think about the nod of your head, the rows of an orchard, the artistry of petals. Patterns can be pleasurable.
In language, rhythm and repetition are often used as the building blocks for poetry, there's the rhythm of language created by syllables and their emphasis such as in Shakespeare's sonnet 18. There's repetition of language at multiple levels, the repetition of letters, "breathe", "see", "thee" and of other words. With so many uses repetition is one of poet's most malleable and reliable tools. It can lull or lift the spirit, amplify or diminish the line, unify or diversify ideas. In fact even rhythm itself, a repeated pattern of stressed syllables is a form of repetition. Yet for all it's varied uses too much repetition can backfire. Imagine writing the same sentence on the blackboard twenty times again and again or imagine a young child clamoring for his mother's attention mom, mommy, mom.
This is not exactly what we might call poetry. So what is poetic repetition and why does it work?
Possibly most familiar is rhyme, the repetition of like sounds in word endings. As with Shakespeare's example we often encounter rhyme at the ends of lines. Repetition in this way creates expectation. We begin to listen for the repetition of those sounds, when we hear them, the found pattern is pleasurable. Like finding Waldo in the visual chaos, we hear the echo in the oral chatter. Yet, rhyme need not surface solely at a line's end. The repetition of vowel sounds is called assonance and can also be heard in songs.