Why is the meaning of life 42?

It's just a well know joke made by the people who made the Hitch-Hiker's Guide To The Galaxy! When you ask someone, 'What is the meaning of life?' they'll say '42' because they can't be bothered to think of the answer! "Which would probably take about a billion years!"

Also, the meaning of life is 'to die'. This can be interpreted to mean 'two die' (as in dice). The total number of spots on two die is 42. This is the most logical explanation.
That's just something Douglas Adams made up in the book "the hitchhikers' guide to the galaxy".