“A difference of opinion is what makes horse racing and missionaries”

Samuel Langhorne Clemens (1835-1910), using the pen name Mark Twain, wrote in the book The Tragedy of Pudd’nhead Wilson (1894):
 
“It were not best that we should all think alike; it is difference of opinion that makes horse-races.”
 
The saying was soon used for other things, such as “a difference of opinion makes horse racing and boxing” or “a difference of opinion makes horse racing and politics.”
 
The humorist Will Rogers (1879-1935) wrote a memorable variation in a 1933 newspaper column:
 
“A difference of opinion is what makes horse racing and Missionaries.”
 
       
Wikiquote: Pudd’nhead Wilson
Pudd’nhead Wilson is an 1894 novel by Mark Twain.
(...)
It were not best that we should all think alike; it is difference of opinion that makes horse races.
. Ch. 19
 
Google Books
The Tragedy of Pudd’nhead Wilson
By Samuel Langhorne Clemens (Mark Twain)
Hartford, CT: American Publishing Co.
1894
Pg. 197:
It were not best that we should all think alike; it is difference of opinion that makes horse-races. — Pudd’nhead Wilson’s Calendar.

Google Books
June 1896, Reliable Poultry Journal, pg. 321, col. 2:
It Is difference of opinion that makes horse racing, and should a day dawn in our life-time when men are agreed in all things, we shall immediately ask for a passport to a better world than this one would then be.
 
30 October 1899, Philadelphia (PA) Inquirer, “The Old Sport’s Musings,” pg. 6, col. 1:
Difference of opinion makes horse racing, and it makes big fights possible.
   
3 February 1900, Cleveland (OH) Plain Dealer, “A Pernicious Form of Betting,” pg. 4, col. 2:
“It is difference of opinion that makes horse racing,” and difference of opinion is also likely to make betting.
 
Google Books
May 1908, The Railway Clerk, pg. 215, col. 2:
... and though some of our members are of the opinion that two factions In a lodge are conducive of no good results, yet as Mark Twain once said, and truly, “It were not that we should all think alike; it is difference of opinion that makes horse racing.
 
14 November 1909, Denver (CO) Post, “Election in Motor Club Absorbs All Interest” by Wirt McCarty, sec. 2, pg. 2, col. 1:
But, as difference of opinion makes horse racing and politics, so a difference of opinion as to what is best for the local club may see some stirring times in the political pot of the organization, which pot has already started to boil.
   
Google Books
November 1917, The Bookman, pg. 356, col. 2:
There is a Kentucky proverb to the effect that “a difference of opinion makes horse-racing”; and it is a difference of opinion that makes criticism entertaining.
 
28 May 1920, San Diego (CA) Union, “Another Attorney Heard on Ireland,” pg. 4, col. 4:
Editor San Diego Union: I am aware that it is difference of opinion that makes horse racing and the practice of law possible.
 
9 July 1923, San Diego (CA) Union, “Tierney Improving Is Word from L.A.” by Right Cross, pg. 9, col. 1:
It is differences of opinion that makes horse racing and boxing popular with the public.
 
5 February 1928, San Diego (CA) Union, “Rich Mixtures” by Bud Landis, Automotive Section, pg. 1, col. 8:
Difference of opinion makes horse racing and is handy for starting family rows.
   
18 October 1932, San Diego (CA) Evening Tribune, pg. 12, col. 1:
THIS S. D. VISITOR
DIFFERENT; HE’S
HURRYING BACK
TO COLD CLIMATE
Difference of opinion makes horse racing and elections interesting.
 
Google News Archive
16 April 1933, St. Petersburg (FL) Times, “The Missionary Has an Inning” by Will Rogers, pg. 4, col. 5:
A difference of opinion is what makes horse racing and Missionaries.