Why do we say "One man's meat is another man's poison"?

Meaning:

Everything is relative - what one person values, another may think worthless.

Background:

This expression originates in antiquity. Whether the Roman poet and philosopher Titus Lucretius Carus (known as Lucretius) coined the expression in the first century BC, or merely repeated it, his is the oldest known reference: "quod ali cibus est aliis fuat acre venenum" (what is food for one man may be bitter poison to others).

The oldest reference known in English is in the autobiography of English composer Thomas Whythorne (c. 1576).

By the early 17th century the expression is clearly well in use as Jacobean playwright Thomas Middleton writes "Whereby that old moth-eaten proverb is verified, which says, 'one mans meate, is another man's poyson'" (1604).

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