"Y's a Crooked Letter and you can't make it straight" - so I was told whenever I started to ask awkward questions. However Why? is also for me the most interesting of questions, since the intelligent asking of it, leads to insights into the minds of those who are probably far more insightful than I.
Who?, what?, when? and where? pale into insignificance because their answers are often mere bland nouns while how? begs to be answered with a verb - a simple doing word to be thrown down - a bone with which I should be happy.
But WHY is the important one. It screams to be answered with "because..." followed by a gusher of wonderful insights and light shed into my darkness which is going to make me a wiser and better person. I love Why? because it is also the most difficult question - it is the question that people wish you wouldn't ask, because in answering it they come to have some insightful moments which maybe point to some of the really really stupid things they have been doing and saying - and they know that you know it.
I love Why? It puts people on the spot - but more importantly it puts myself and my preconceptions on the spot and allows me to see a better way.