Why is it that people do things but then when asked why they did it, they don't know why they did it

Why is it that most people do something, whatever, say something or do something and then when asked "Why did you do this when you just said yourself you could have done that, or this" they say "I don't know". Then you can ask "Well, how can you do something but not know why you did it? Can't you just remember what you were thinking when you did it and then explain why you did it?" and they say "No".. I have wondered this for years and have never found a suitable answer. Help!

There can be various reasons for this. Sometimes people do stupid things and only realise how stupid their actions were after they come face to face with the consequences. They probably often ask themeselves the same question you're asking now. That's normal behaviour though, nobody's perfect, everyone makes mistakes, some learn from it others don't.
Other times people do actually know the reasons behind their actions, but these reasons may seem strange and so, in fear of being judged, they don't reveal the truth, but instead pretend to be confused.
Pressure is another reason. Sometimes people are almost forced to do something because of pressure from someone else. People who don't know the reason for their doings are usually easily convinced to do things which they later regret.
Hope I answered your question :]
I'm basing this on a psychology class on human behaviour by the way.

I think thats just a human thing. I do that all the time.