Well, for me it'd be: "Why don't you climb Mount Everest?" "Because it's there!" (Last week I saw a pretty good film about mountain-climbing called "Touching the Void" - confirming me in my resolution that they aren't getting me up on one of those things. High hills you can go rambling across are one thing, but vertical mountains with crevasses and that are quite another. I'm keeping my feet on the ground.)
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True, greg - "because I was a randy middle-aged man" is indeed the missing bit from Bill Clinton's answer. (Or at least part of it - just because you like a drink it doesn't mean you steal them when the bartender's not looking.)