Variety is the spice of life

Can anyone tell me anything about the proverb 'variety is the spice of life'?

It dates at least from the 18th century, having being cited by the poet William Cowper in 1795.

Earlier variations:

First century B.C. No pleasure endures unseasoned by variety. Publilius Syrus. Maxim 406.

1758-60. The joy of life is variety; the tenderest love requires to be rekindled by intervals of absence. Samuel Johnson, The Idler.

1785. Variety's the very spice of life, That gives it all its flavour. William Cowper, The Task.

From Random House Dictionary of Popular Proverbs and Sayings by Gregory Y. Titelman (Random House, New York, 1996). Page 351.