Here is a list of Theories I have heard:
Tribute to Jackie Robinson
Double blackjack(2x21)
Number of lines on a page
Tribute to Lewis Carrol
Does anybody know the the true reason why Douglas Adams picked this number?
I think it’s in a forward to one of the versions of a HGttG collection. (I can’t find mine, dang it.)
Basically, it sounded funnier to him than other numbers, if I’m remembering correctly.
Feel free to correct, contradict, or castigate me. 
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Jokes in base 13, &c.
Nice username. 
Because it’s what you get if you multiply six by nine.
Because he thought it was (obviously) of no significance.
Yes, the answer to life is Jackie Robinson. It all makes sense now.
Only if you are working some strange base that isn’t base 10.
It was just fortuitous.
Another thing I just realized, there are 42 gallons in a barrel of oil.
If you’d read the books, you’d know that 6 x 9 = 42 is another Douglas Adams joke.
Simply, the reason for anything in Douglas Adams’ books is that it’s a joke.
I saw one recently that was a twist on this, it said
“Math is the meaning of life
M is the 13th letter
A is the 1st letter
T is the 20th letter
H is the 8th letter
13+1+20+8= 42”
I thought it was at least clever.
Douglas Adams has remained funny even in death.
If you think there is, or should be, an explanation, you haven’t got the joke.
…Well, why wouldn’t it be?
I always knew there was something fundamentally wrong with the universe.
That’s it precisely. In an interview, Adams said:
I prefer to think he foretold the coming of Darren Milane
Regardless of what Adams may have said, I don’t think it’s coincidental that Lewis Carroll used 42 a LOT in the Alice books (and in Hunting of the Snark as well.) I like to think that may have been floating around in Adams’ head, unconsciously. Especially since Carroll’s writing, while full of jokes and humor, has a bleak metaphysical underbelly… hence, Life.
Because 42 / 3 = 14.
And as any fule kno, there are 14 k of g in a f p d.