I'll add that another common intensifer here is "Why the heck* did you do that?" or "What the heck did you do that for?"
*'Heck' is a bit old. It's used to avoid "What the hell" which offends some people, and 'What the f***", which I would advise everyone to avoid. But you probably should know what 'WTF' means, since you can't avoid seeing it on the 'net. By itself, this is used as an expression of surprise, outrage, etc. In a sentence, it means the same as above. I don't like any use of the 'f***' word, and am only writing this for comprehension purposes.