A dictionary of Proverbs (Oxford Paperback Reference)

A DOG is for life, not just for Christmas
The slogan of the UK’s National Canine Defence League, introduced in 1978 with the intention of dissuading people from giving puppies as Christmas presents. Widely quoted in its original form, it has also spawned any number of humorous by-forms.

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  1998 R. RAY Certain Age 344 A tree is for life, not just for decorating with small shiny objects. 1999 Jewish Chronicle 30 July 42 And remember, a pet is for life, not just for Rosh Hashanah. 2001 Country Life 29 Nov. 96 (advertisement) A Billiard table is for life not just for Christmas. ■ constancy and inconstancy
  

The DOG returns to its vomit
The expression is frequently found in various metaphorical and allusive forms, such as to return like a dog to his vomit, as illustrated below. Before 1534 (see quot.), the proverb is used in more or less similar forms in earlier versions of, and commentaries upon, the Bible. The concept enjoyed wide popularity in the Middle Ages. The biblical passages involved are: PROVERBS xxvi. 11 (AV) As a dog returneth to his vomit: so a fool returneth to his folly; also 2 PETER ii. 22 (see below).

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  c 1390 CHAUCER Parson’s Tale 1. 137 Ye trespassen so ofte tyme as dooth the hound that retourneth to eten his spewyng. 1534 W. TYNDALE tr. Bible 2 Peter ii. 22 It is happened vnto them accordinge to the true proverbe: The dogge is turned to his vomet agayne. 1832 S. WARREN Diary of Late Physician II. vi. His infatuated wife betook herself—‘like a dog to his vomit’.. —to her former.. extravagance and dissipation. 1981 P. McCUTCHAN Shard calls Tune xvi. The old saying that the dog returns to his vomit, the criminal to the scene of his crime. 1993 G. LANDRUM Rotary Club Murder Mystery 44 ‘You know what the Bible says,’ Harriet replied. ‘ ‘“The dog shall return to his vomit and the old hog to his wallowing in the mire.” ‘ "habit; human nature