My favorite thing to tell my four year old when he asks "why did you do that" is...
It seemed like a good idea at the time. I tend to live my life that way. If it seems like a good idea I do it... if it doesn't work out..
oh well, it seemed like a good idea at the time.
You can apply that statement to just about anything you have done, and its true. If it didn't seem like a good idea at the time, you probably didn't do it...

Since you did it.. It seemed like a good idea at the time. Even the stupid things you do... if you think hard enough.. they did seem like a good idea at the time... You wouldn't get caught, you would impress somebody, nothing could go wrong... It seemed like a good idea at the time didn't it?
"Dad, why did you cook green beans?"... "Why did you go right?"... "Why are we going to X?".. My answer: "Seemed like a good idea at the time". What I'm really waiting for is for him to use it on me when I question why he did something. It will just crack me up
The day he does that I'm just gonna have to laugh...
Laters,
-d.