The Slate article linked on page 1 has a pretty good theory on why he does this: basically, he’s a fictional celebrity endorser and it would undermine his status as the most interesting man in the world if he were to gush about XX.
It seems obvious to me. He may drink martinis some of the time or even Singapore Slings if the situation calls for it but, when it comes to beer, it is always going to be Dos Equis. The most interesting man in the world would never drink the same thing all the time. That is trailer trash material.
Folks come from miles around just to hear him fart. There is no smell, his farts don’t stink.
He once won the World Series of Poker with a pair of deuces, a three of diamonds, a six of clubs and an Uno card.
His urine was used in the formulation of a special edition Dos Equis triple IPA that prompted the Nobel Foundation to create an award for Brewing Arts.
Dos Equis is actually carbonated with his farts.
The president of American Express never leaves home without HIS card (although he sometimes forgets his Amex).
Someone suggested he write down his dreams when he wakes up. He tried it once, and the result was on the New York Times bestseller list for three years.
He can recharge his cellphone battery just by holding it.
And then put it back together, just by squeezing the two halves in one hand. In the process, he realigned the facets into a hitherto-unknown pattern that increases radiance and sparkle to double the previous cut.
He can recite the human genome.
And his excrement smells like freshly baked cinnamon buns.
He is the reason Polar bears are left-handed.
His biography is found under the Reference section in the library.
His poetry is found in the Reference section in the library! 
Yup…I went out and bought some Dos Equis just because I liked the commercials so much. 
Such commercials should be rewarded!
There’s a feature in Microsoft Word that was put there just for him.
That is such a geeky, subtle and bad joke that it is really funny.
I though my 7/11 prime number one was better…but I’ll take any compliments I can get
I will note that he IS such an interesting man that the universe will die a heat death before he ever sends me a note of thanks…
And it even WORKS !
Not that he would actually stoop to using Word. He writes all of his own software, one bit at a time.
He won the Kentucky Derby on a chihuahua.
He donated both of his kidneys. He doesn’t need them.
He tried out for the leading role in Hung, but was over-qualified.
Michael Bay wanted to direct his life story. But he felt Bay’s directorial style was too understated.