Why do we say "One good turn deserves another (or quid pro quo)"?

Meaning:

A favor should be returned in kind, e.g. "you helped me with my homework last week, so I'll help you with yours this time". The maxim traces back to at least 1400.

Background:

In Latin, the concept is expressed as quid pro quo (meaning "this for that"). Quid pro quo is used both in colloquial English and as a legal concept in the area of trade. For example, a contract is said to be binding if it is quid pro quo, which is to say that it involves an exchange of goods or services for something of comparable value - whether that be money or other goods/services.

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