It seemed like a good idea at the time
"It seemed like a good idea at the time" is the response statistically most likely to follow the question: "Why did you do something so stupid?" It may be used as an expression of regret, bemusement or disinterest, but the event to which it refers will always have been foolish, impromptu, impulsive or badly thought through, often resulting in a catastrophe that most rational and/or sane people would (usually) have been able to predict.
Almost always used in the past tense, the pinnacle of its usage (and accompanying philosophy) is when one realises before any event occurs that "It seemed like a good idea at the time" will be the perfect description, later on, of what is about to happen. It goes marvellously with the famous Darwinian saying, "Hey everyone, watch this!"
It is also a great Canadian Comedy.