"Forty-two" is "the answer to life, the universe and everything." Now that Douglas Adams is no longer available for questioning, only Stephen Fry knows why... and he's promised not to tell.
With regard to the why 42 bit, Lewis Carroll is the answer. There was a programme on Radio 4 some time ago looking at the work of the producer of the original radio series of Hitchhiker. He said, when talking about Hitchhiker, that he'd suggested 42 because the humour in Hitchhiker and Carroll's works was similar. Carroll had a thing about the number 42, and it does crop up in a few of his works.
lol ... Carroll did come up with an answer to the "Why is a raven like a writing desk ?" riddle, though he did add "This, however, is merely an afterthought; the Riddle, as originally invented, had no answer at all".