Don't you hate that question? We ask our kids the same thing and inevitably the answer is either, "I don't know" or "Well, I thought it was a good idea at the time." In all honesty, the answer to this question is not really going to stop us from making the same choice or mistake again.
There are three questions that I ask after completing a task or when considering whether to make the same choice again. These three questions help me look more thoroughly at the actual results versus the perceived result I thought would come to fruition.
Question 1: "What three things went so well that I would do them again?"
Question 2: "What three things could use improvement or did not go as planned?"
Question 3: "What did I learn from this activity/choice/task?"
Answering these questions will usually give me more insight than the answer ever would to "why did I do that?" What question do you ask yourself to determine the success or failure of a choice?