Why do we say "Ships that pass in the night"?

Meaning:

People who meet once and form a close bond, but never meet again

Background:

This saying originates from Henry Wadsworth Longfellow's poem "The Theologian's Tale", which forms part of Tales of a Wayside Inn, a collection of poems narrated in the voices of a group of people in the tavern of the Wayside Inn in Sudbury, Massachusetts - all contemporaries and friends of Longfellow. Although the narrators are not identified, many are so clearly characterized that they can be identified.

The Theologian's Tale is clearly identified as written from the point of view of Elizabeth Haddon (1680-1762). Haddon was born in England and set sail to the New World in 1701 as a single woman without family. Shortly after her arrival she made a marriage proposal to John Estaugh, a Quaker minister, and they were married in 1702. Their courtship is narrated in "The Theologian's Tale", also known as "Elizabeth":

Ships that pass in the night, and speak each other in passing,
Only a signal shown and a distant voice in the darkness;
So on the ocean of life we pass and speak one another,
Only a look and a voice, then darkness again and a silence.

"The Theologian's Tale" ("Elizabeth"), Stanza IV

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