Why do we say "One Swallow Does Not Make a Summer (or Spring)"?

Meaning:

A single instance does not indicate a trend.

Background:

This expression is attributed to Aesop but it is not clear whether it and the related fable date back to Aesop or whether one or both are later additions.

What is clear is that the expression was in use in the middle of the 4th century BC, about 200 years after Aesop is believed to have lived, as it is found in Aristotle's Nicomachean Ethics, a series of ten books, originally ten scrolls, which are believed to be based on notes from his lectures at the Lyceum. In this, among other topics, he explores the elements of happiness and, having listed the attributes, concludes that to be happy takes a lifetime because one swallow does not make a spring.

The fable goes something like this:

A young man spends all his money on gambling and luxurious living until he has just the cloak on his back to keep off the cold weather. Seeing a swallow fly past on an unusually warm day, he concludes that spring has come and sells his cloak in order to be able to place one more bet. Not only does he lose the bet but the weather turns cold again. He finds the swallow frozen to death and blames it for deceiving him. In some versions the young man also dies.

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